filmtrain, I really wouldn't worry about how the train looks on 42" curves. The wider 72" curves look more "realistic", but that's a relative concept with tinplate, and it really only becomes a consideration on a permanent layout. For a Christmas setup, trains on the 42" curves will look like a toy train, which is perfect.
12" is certainly not too tight to fit two tracks side by side. As Owen says, check your swings where you enter and exit curves, and you'll be fine.
24' straightaway is a GREAT stretch, that will be very fun to run!
