Fairly clear cut that the original person should take a bigger share. One thing I would preface that with is that with is that if the cofounder comes in and fundamentally changes the business into something much bigger/ better than it was they are entitled to equal or close to equal.

I think the hard case is when you start off with equal and then down the track one founder has a different level of enthusiasm and expectation on level of time involved. If down the track you are working long ours and just about every day and your cofounder is treating it as a 9-5 there is bound to be friction.