Got a buddy pass question for everybody...

When booking a connecting flight, can you book a ticket using co-terminal airports? For example, I have a friend using a pass from ORD - JFK - AMM. If the flights sell out to JFK, can I book the pass ORD - LGA and JFK - AMM? The Shuttle runs between ORD and LGA, so it's much easier than trying to get on one of the four RJ's to JFK. I think the computer will let me book the pass this way, but I don't want to get into trouble. Thanks!