You can use two chainrings as long as they are close together and you have long dropouts, but you will not be able to shift on the bike.

I rocked the "Dingle Speed" (as someone around here called it) for a little while with a 42 ring on the outside and a 40 on the inside. I didn't have single chainring bolts. It turns out you can ride just fine, even without the perfect chainline.

Check out the Quickbeam here.