This answer is very long but it suggests six possible answers, which I hope will be enough.
I checked the list of That's so Raven episodes.
"Double Vision" aired on 17 December 2004. It is listed as being the 7th episode of the 3rd season and the 50th overall episode and having the production code 318 indicating the order in which it was produced.
"Vision Impossible" aired 6 January 2006. It is listed as being the 34th episode of season 3 and the 77th overall episode, and having the production code 335. Note that 34 - 7 = 27, but 335 - 318 = 17.
The episodes, as is very common, were not originally broadcast in the order they were produced. As another example, the last 3rd season episode broadcast, "The Four Aces" is the 35th episode of the 3rd season and the 78th overall episode, and has the production code 331 which indicates it was produced 3 episodes before "Vision Impossible", which has the highest production code, indicating "Vision Impossible" was the last episode produced.
And the Wikipedia entries on "Vision Impossible" and "The Four Aces" say:
Note: The episode has a scene in the episode "The Four Aces," which was a display error, since the episode "The Four Aces" only aired ten days after this episode.
Note: Even though "Vision Impossible" aired before this episode, a scene from this episode is seen in "Vision Impossible".
So apparently the fictional date of "Vision Impossible" is AFTER the fictional date of "The Four Aces", even though "Vision Impossible" aired BEFORE "The Four Aces".
It is clear that "Double Vision" is earlier than "Vision Impossible" in the two main indicators, broadcast date and production code, and so probably happened earlier than "Vision Impossible". If "Double Vision" happened before "Vision Impossible" Raven couldn't tell Ben in "Double Vision" about any of her experiences in "Vision Impossible" since "Vision impossible" hadn't happened yet. And so Raven couldn't tell Ben about any psychics she met for the first time in "Vision Impossible", since she hadn't met them yet.
Raven met other teen psychics and their mentor Dr. Sleevemore in "Saving Psychic Raven", broadcast on 21 February 2003, the 8th episode of the first season, and the 8th episode overall, and with the production code 120 indicating it was probably the 20th episode made.
So "Saving Psychic Raven" should happen earlier than "Double Vision" since it was produced earlier, and in an earlier season, and was broadcast years earlier. So Raven could have told Ben about meeting the other teen psychics and their mentor Dr. Sleevemore, and it seems like an obvious thing to do, to tell another teen psychic you have discovered that he and you are not the only two.
I can think of three possible reasons Raven might not have told Ben about the other psychics she knew about.
1) Possessiveness. Raven might want to keep Ben as a friend for herself instead of sharing him with the other teen psychics she had discovered, even though Ben is never seen in any later episode.
2) Protectiveness. Raven might want to keep Ben away from the other teen psychics she had met if she considered them to be bad influences in any way.
3) Carelessness or forgetfulness. The qualities that make Raven Baxter a funny character are the qualities that make her a less than perfect person. Raven often acts very goofy, careless, thoughtless, and excitable. Possibly Raven didn't think enough about anything that she might do for her new psychic friend Ben, or maybe Raven decided to tell Ben about the other psychics but kept forgetting to do so and finally forgot about it totally.
And there is a fourth possibility:
4) Maybe Raven did tell Ben about the other psychics during "Double Vision" or afterwards - off screen.
And here are two other possibilities:
5) Even though "Saving Psychic Raven" was produced and broadcast before "Double Vision" in our world, it might not have happened before "Double Vision" in the fictional world of the Disney Channel Live Action Universe or DCLAU.
As far as I know there is no legal or natural rule forcing producers of TV shows to depict events in the same order as they happen, especially if those events happen in some alternate universe that viewers and critics don't know how to reach to check on the authenticity of the shows. Considering how much the creators of history based shows change and rearrange events despite them being recorded, why should creators of TV shows depicting events in alternate universes worry about people complaining about the order of events, when the creators are the only ones who know anything about those alternate universes?
And so there is no reason to be absolutely certain that "Saving Psychic Raven" happens before "Double Vision". If episodes might be produced and broadcast in a different order than their events happen, there would be no way to tell which episode happens first. And some people might consider Raven not telling Ben the important fact that she meets other psychics in "Saving Psychic Raven" to be strong evidence that "Saving Psychic Raven" happens after "Double Vision" and not before "Double Vision".
Thus it is possible that Raven meets the other psychics in "Saving Psychic Raven" sometime after "Double Vision", and so cannot tell Ben about meeting them.
6) As I said, That's So Raven and Raven's Home are part of the Disney Channel Live Action Universe or DCLAU, which is quite obviously in a different alternate universe to our universe.
WARNING! SOME OF OF THESE LINKS CAN LEAD TO HOURS OF TIME THAT SOME PEOPLE WOULD CONSIDER WASTED. YOU HAVE BEEN WARNED.
Furthermore, some people suggest that the DCLAU is part of the John Munch or Tommy Westphall Universe, which contains between over 100 and over 400 TV series according to various sources that have different standards for including shows.
But for the moment forget about the DCLAU and imagine that That's So Raven and Raven's Home are totally separate from all other books, comic books, movies, and TV shows. That's So Raven and Raven's Home will still have to happen in an alternate universe to our own, just as most fiction happens in alternate universes. As nearly as I can tell most of the characters live in contemporary Chicago, which means it would be very easy for people with the right techniques to find proof of the existence of those characters. Because such proof cannot be found, those characters must not exist in our universe.
Therefore, whenever a view suspends disbelief in those characters, that viewer must for the moment sort of believe that they do exist, but in some alternate universe.
But why should there be only two alternate universes in existence, especially since the many world interpretation of quantum mechanics, the only scientific justification for alternate universes that I know of, postulates that countless alternate universes branch off every second. Thus there should be countless thousands and millions and billions of alternate universes where Raven Baxter and the other characters were born and met each other.
So why should the creators of That's So Raven and Raven's Home restrict themselves to making TV depictions of events in only one of the countless alternate universe of Raven's life, when they can chose from the most entertaining events in many different alternate universes?
Note that after the events of "The Falcon and The Raven", June 25 and 26, 2018, Raven and her family will probably be as doomed as George Armstrong Custer and his men 142 years earlier, or at least as messed up as Reno and Benteen's men on those dates in 1876. Norman would probably sue SCUT and Raven for the loss of Ernesto, and SCUT will probably fire Raven and sue her. Raven will probably lose her driver's license, perhaps permanently, for her dangerous erratic driving, and face civil and even criminal proceedings.
So Raven and Chelsea's household will probable face financial ruin and break up. Therefore later episodes where everything is fine should happen in alternate universes where Norman and Ernesto never got in Raven's car, or where Raven behaved reasonably. Later episodes where Raven knows about Booker's psychic visions will be in alternate universes where he had other reasons to tell her about them, reasons unconnected to the events in 'The Falcon and the Raven".
If "Saving Psychic Raven" and "The Four Aces" and "Vision Impossible" episodes of That's So Raven and the upcoming Sleevemore episodes of Raven's Home all happen one after another in one timeline, that is enough for one alternate universe. Therefore "Double Vision" is probably in another alternate universe, making the reason why Raven didn't tell Ben about the other psychics the fact she never met those other psychics in the alternate universe of "Double Vision".