I normally don't order two of something, but I will on occasion and I don't think twice about it. Escargot is often a double for me. Sweetbreads and anything with mussels in the shell too.
Some lines won't offer a traditional surf and turf plate, but instead have the lobster as one entree and the prime rib as a separate entree. In those cases the wait staff anticipates people wanting both and will often offer to put a lobster tail on the prime rib entree plate. If not, then ask for both plates.