i have a 1972 schwinn deluxe twinn. i bought it all in boxes, not sure which way the chains go on. its got a small single sprocket on the front and 2 sprockets on the rear with a five speed cassette. i think the front chain goes on the the small inside sprocket on the rear and the rear chain goes on the outside large sprocket then goes on the cassette. but when i have it that way and shift to the largest cog on the cassette the chain crosses so much that it pulls it off the sprocket. what am i doing wrong? thanks

Probably best to post some pictures. Always makes this kind of thing easier.

Use google images and look at what you have in comparison to other bikes.
Basically the bike is all RH drive.
There should be two chainrings for the stroker. the small and the captain's single small ring get connected. there should be a chain tensioner between the two in the way Schwinn engineered these bikes. It mounts off a bracket under the Boom tube.
The large ring at the stroker is the drive ring for the hub sprockets.
5 speeds are sensitive to chain-line and bent chainring teeth. The final drive sprocket should align with the center of the freewheel if you applied a straightedge to it and extended it back over the freewheel, the chainguard must be off for this evaluation.

Some old French tandems placed the final drive at the front chain set to reduce the angles of attack between the freewheel and chainrings, this bike was not designed that way.