Hi,
I would do the following:
1 - connect one board
2 - start a ccs debug session, having selected "TI MSP430 USB 1"
You now know that session is attached to that board. Then (keep the debug session live)
3 - connect board 2
4 - launch another ccs workspace/session (note you need a different workspace)
5 - start a ccs debug session, having selected "TI MSP430 USB 2"
This should get you working with two debug session on two msps at the same time.
Best Regards,
Lisa