That error is expected in that case: you can't run two Orbiter instances
at the same time; you will need to run them one-at-a-time.
Out of curiosity, why do you need to run both instances at the
same time? It's not like you can easily fly two vessels in two windows at once, and even if Orbiter allowed running multiple simultaneous instances, it would certainly adversely affect the frame rate of both instances.