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.
About multiple Orbiter instances, let's not forget what kamaz made some years ago:
https://www.orbiter-forum.com/showthread.php?t=28826
https://www.orbiter-forum.com/showthread.php?p=241888&postcount=13
I guess it won't work on O2016, at least I didn't try it...