I'm attempting to learn how to commentate, and I agree with most of what DrDogg is saying. However, let me throw my own 2 pence in.
1: Fighters are too fast to do a literal play by play, you can't speak fast enough. The closest you can do is commentate on changes on match state (offensive shift, specific choices a player makes (especially uncommon ones like ending combos early), sudden reversals and why those reversals work) or on the current match state (spacing, fireball wars, corner pressure).
2: Hype is also important in keeping the audience interested. You want your audience on the edge of their seats. There are various ways of doing this, each person has their own method and those methods have varying degrees of success, depending on the audience of course.
3: In terms of teams, I think it's best with two people at most. If there are two people commentating they need some sort of dynamic where they bounce concepts off of each other. (For example, if one of them plays the character defending from a blockstring the other commentator could ask that person what viable ways that character has to get out.)
4: A one person team is doable and sometimes I think preferable for fighting games. With two people the matches go by so quickly that the subject that they were discussing one moment will be outdated within a few seconds. This is more true for games that have a faster pace.
In terms of "hype" commentary, as I call it, it has it's place. Primarily, when all the people involved are just goofing off and the match is semi-serious (or serious but not as serious as tournament matches). I like this kind of thing now and again but having all commentary like this would end up getting annoying rather fast.
As for online, it isn't impossible to commentate online matches, but the content of the matches would be questionable due to the input lag. The players would make decisions that they would not necessarily make against offline opponents and it changes way the game is played. As long as you are willing to deal with that, online matches are fine. But if you want something that would be comparable to tournament footage then you need offline matches because they lack several variables that online matches have.