This should work. Smoke your baby backs (need smaller ribs than full spares so they finish within 4 hrs or so) at 250*. Slightly higher temp cause you want the second batch to finish while the first batch is still hot in the cooler.
So if you hold a batch for 4 hrs or so, throw the held batch on the grill with the batch right out of the smoker (lightly glaze the second batch like Tom suggests before you uncooler the first batch and lightly glaze them). You should wind up with both batches being fairly similar once they both pick up heat from the grill. Taste test the two versions with your friends and see if they can detect a difference. It will make for some fun, and all the while, they'll think they've died and gone to hog heaven.
I've got the same situation. I want to smoke ribs Memphis Style for my friends, but I've got more friends than smoker space (Elite).
Let us know how they turn out.