I would definitely recommend that you pick someone up front to be a Project Manager. Your friend might be this, as you described his role as "finding clients" and "some admin etc", but it's really a bit more than that - you might want to bring in a third founder with experience in running projects.
I'll echo what others here are saying - don't split things 50/50 (or by any percentage, really). Everyone on the project should have a bill rate.
I'm a consultant on a few projects that are structured this way, and it seems to really work well.
And then one of you messes up on their part and either the billed hours total is more than the revenue or the client refuses to pay so much (depending how you're charging) ... what now?
Also billed hours means that there's an incentive to work slowly to get a greater share out of the business.
I'll echo what others here are saying - don't split things 50/50 (or by any percentage, really). Everyone on the project should have a bill rate.
I'm a consultant on a few projects that are structured this way, and it seems to really work well.