Now that is a very good question, to which I don't have an answer. I would be curious to know airlines' answer to that (it may vary by airline). If you don't get miles for both tickets, can you at least get checked baggage allowance for both? I think you could make a good argument for it.

Now that the checked luggage allowance has been reduced on most airlines to 50 lb/piece, this is a relevant topic. I know a lot of people traveling to/from Europe who counted on that higher 70 lb/piece allowance.