According to general understanding, we can only really concentrate on one thing at a time. But I know that this isn't actually true.
There are two of you (at least!) inside your head - a left hemisphere and a right hemisphere. Most of the time these two sub-brains work together. But our two thumbs are quite capable of doing quite different things from one another. Probably the most interesting music on kalimba happens when the two thumbs are working independently, but in a coordinated manner.