Before co-founders do any significant work on a startup together, they’d be well-advised to put something in writing about how they plan to work together and what their respective roles, ownership, compensation, etc., will be. Otherwise, things could get really messy if the co-founders were to get into a dispute later, especially if they decided to split up.
Take a look at the Founders’ Accord, in essence a co-founders’ pre-nup, published by a small NYC law firm. Of course it’s not a substitute for legal advice, but it seems to be a useful resource, and it’s almost certainly better than not having any kind of written co-founder agreement at all.
I’ve added a link to the Founders’ Accord to the Legally Lean cheat sheet.