Firstly, people in different parts of India speak different languages, so, If a movie is made with reputed cast and crew then the producers are likely to dub the movie in different languages such that people all over India would enjoy watching movie in their native language. Also remember that the movies with high budget are the ones mostly get dubbed in different languages to get good money collections at box office.
Now, as per your question, you were surprised that the movie is released with different crew in two different languages mainly Tamil and Hindi and also asked whether if it is actually common practice in Indian cinema to simultaneously make a movie in multiple languages.
My answer, I would not say it is a common practice but definitely it's not surprising for us in India. Let me give a quick brief about the director of the movie. Mani Ratnam born in Tamil Nadu (a state in south India) but his film making is admired by the whole nation. His movies are made in one language and dubbed in different languages.
The movie Raavanan (tamil) and Raavan (Hindi) is simultaneously released. As you mentioned, the actor of the tamil movie Raavanan is Vikram, he is very reputed in South India for his acting skills and South Indians would love to see Vikram in a dominant role in Raavanan rather than a Hindi actor Abhishek Bachchan who also happens to be famous for good acting skills. Similarly, the same thing applies for the North Indians. This may be the reason why Raavanan with Vikram is made in Tamil and dubbed in Telugu and probably Kannada and Malayalam (native languages for Southern Indians) and Raavan with Abhishek Bachchan is made in Hindi. So, it is in his best interest to entertain both North and South Indian audience. He made few of his movies with this strategy.
In an interview, he speaks about how he felt making the same movie with different cast.
Directors normally find it a nightmare to juggle just one cast, and here you have gone made the same movie twice, in Hindi and Tamil. What was your experience?
I had two nightmares (laughs). I have not done this before, and the
two movies were shot back to back, not simultaneously. I dealt with
two different set of actors, and each brought its own changes.
Filmmaking brings its own organic way of doing things, changes, it
develops a character of its own.
Dil se is a movie he made in Hindi and dubbed in Tamil.
Did you resort to this because in Dil Se/Uyire, the same cast speaking
in two different languages did not gel with the audience, and that the
north and south have their own sensibilities?
Dil Se was dubbed into Tamil as Uyire. Dubbing a movie brings its own
set of compromises, and you end up losing some of the elasticity.
your another question,
if those versions are really made exactly equal or if there are larger-scale differences in the story catering to the specific demands of the respective cultures
In some cases, even though the story line remains exactly the same, but their might be differences in the different versions with respect to culture and fashion, examples : dressing style and living style.
In some other cases, consider this movie Ye Maaya Chesave (2010) in Telugu and Vinnaithaandi Varuvaayaa (2010) in Tamil, both directed by Gautham Menon with different cast and released simultaneously.
In Tamil version, the heroine leaves the hero in the second half of the movie and gets married with another guy in the end.
But in the Telugu version, director learned that this kinda ending would disappoint Telugu audience so he changed the ending where heroine patches up with hero in the end and they both get married.
So, you could say that the directors make films with also keeping audiences opinions in mind.