Very common in Europe, and not just at Marriotts. The nicer hotels will at least put a single king-sized fitted sheet around the two mattresses. The really fancy ones have an insert that holds the mattresses together and sort of feels like a single mattress...unless one gets, shall we say, too active with one's partner in the center of the bed.

At least Starwood will usually advertise this fact on their website as "Two Twins: Join as King".