Yes, you can! Save enough for six months to a year in basic living expenses -- that's your seed fund. While you save, keep moonlighting on your project. Your seed-fund will empower you to make the right decision when the time comes (i. e.: When your project gets traction). And you can always do consulting gigs if you need more money.

Well, I just finished working out the terms of the job offer from said other startup and I'm insufferably pleased with my negotiating :-). I probably won't have time for side-projects while I'm there, but even if I don't get rich with them I'll probably walk away with enough savings to set out on my own afterwards.