It doesn't seem Blondie knew Tuco before they met, early in the movie, when Blondie kills the 3 bounty hunters who just captured Tuco.
At this moment, Tuco thanks Blondie but he seems surprised. Then Blondie arrests him and ties him up. Tuco is really upset.
Tuco acts like he doesn't know what does or want Blondie.
As for the point 1., after the first time Blondie frees Tuco from the hanging rope, Blondie shares the bounty with him, half and half. Tuco says
There are two kinds of people in the world my friend. Those with a rope on their neck and people who have the job of doing the cutting. Listen, the neck at the end of the rope is mine, I run the risk, so the next time I want more than half.
Blondie answers that "cutting his percentage might interfere with his aim", to which Tuco replies
But if you miss you better miss very well. Whoever double-crosses me and leaves me alive doesn't know anything about Tuco.
And it happens that the next time... Blondie's first shot is a miss. Tuco hangs from the rope for a few seconds, and he's eventually saved by Blondie. Tuco is angry about the miss, and won't stop complaining. Blondie seems really pissed, stops his horse and says
The way I figure, there is not much future between us.
And Blondie ditches him. This doesn't seem awkward. (Also, maybe there was more than 2 "cutting" episodes that we don't know).
As for 2., I don't recall exactly, but Angel Eyes appears to be well connected and is likely be known by many people.