[Disclaimer: I am not a lawyer. I am especially not your lawyer, or anyone else's. This is not legal advise. I recommend anyone in this situation retain an attorney and ask them for legal advise on this subject.]

Get an angel check. One share in a billion leaves neither cofounder with a controlling interest.

For what it's worth, I recommend giving more like 2% for a sole angel, assuming their contribution runs little further than satisfying this control problem. But technically speaking, whatever somebody agrees to should solve the control problem.