Several hotel programs allow this (Hilton, Starwood). It may be possible to use Hilton to allow miles to be combined, but the exchange ratio AA->Hilton is so bad that it does not seem worth it. With some other airlines where the ratio Airline->Hilton is double that of AA->Hilton, it may be a smaller penalty to transfer miles from the airline to Hilton, and then use Hilton points to get a free airline ticket.
Continental had a unique promotion which allowed miles to be transferred from one account to another for a fee.