I like that approach - very interesting. Have you actually tried it though? I'm just wondering if this is one of those great ideas in theory things or whether real, actual, high-quality folks are saying, "Yes" to this type of deal.
The point is that both I and my friend/co-founder will know what we own and what we have to do, and we should both feel reasonably good about it. I won't feel like I've given anything away, and he will probably feel like he got a fair deal.
If he agrees at the outset and meets all the (negotiated) benchmarks, he will have a legal right to enforce the transfer to the joint company. If he refuses to structure the deal that way, then I will probably interpret his refusal as a sign that he is unfit for the job and possibly even untrustworthy.