If they are offering equity as your payment then I'm surprised that they didn't come with a number. Or maybe they will. It's hard to give a firm answer because it all depends on how much money the two founders are investing on their own. If you say you'll invest equal portions then you can ask for 1/3 of the company. If you're just investing time, how much time, compared to how much of their time plus equity. Depending on those answers, the amount of equity you're worth can drop pretty low.