I don't know if this is in the game or not but I am level 29 and I still didn't find a way to play with my friend
I've always pondered this idea. It could be done but with stipulations. Do you want to play your friend in a live 1v1 battle? Or are you looking to be able to play against one of their teams while it is controlled by the computer?
Playing a live 1v1 against someone may be hard to get going. Each player would want to pick their own team based on what characters the other is using. It may be difficult to be able to finalize a matchup if each person is changing their team up for the best matchup.