In this piece, Christoph Sietzen plays a marimba (wooden keys) and a vibraphone (metal keys). As is common when playing these instruments, he uses four beaters: part of the skill is being able to control the distance between the two beaters in each hand, as well as the amazing hand-eye co-ordination required to play music this fast.
Here is another piece for marimbas, where the performers play drums as well:
Hayes Music blog 