Generally, whoever asked for it first gets to keep it. However, things like RVSM status, standard levels etc will also come into play. Also the downstream effects may come into play, like setting up descent or other aircraft at levels which may cause more conflictions. It's a big jigsaw puzzle really and many factors come into play. These may not be clear to you at the time as the aircraft may be on different frequencies hundreds of miles from each other.
Cheers,
NFR.