[siegel]

Just to throw out another potential concern with a 50/50 or similar split right now - what if your co-founder never decides to join full-time. What will you do then? Will you fire him before he vests in anything significant? If not, then he's potential deadweight on the cap table, particularly from the point of view of potential investors.

There are multiple considerations here. tylercubell is correct that you are taking a risk here that you co-founder isn't. You may never land your first sale. in that case, your co-founder has been earning money at a full-time job and you haven't. Should your equity percentage reflect that?

But let's say you are comfortable with an equal or nearly-equal split, some options to make things more fair, incentivize your co-founder, and make your company attractive to potential investors:

1) Slower vesting for your co-founder or having at least a tranche of his shares only begin to vest when he joins full time; 2) Offering your co-founder a small vesting equity stake now and an option to purchase more that only begins to vest if he joins full time; 3) If you are going to grant your co-founder a large equity stake now, with significant vesting before he joins full-time, the co-founder could grant the company the option to repurchase a certain percentage of shares at cost if the company has its Series A before he joins full time. This would avoid the deadweight on the cap table problem.

On #1 and #2, one core idea is to make the grant of shares now (not down the road) to ensure your co-founder can purchase the shares cheaply, before your common gets priced.