That's very different. If the master is not not connected to a PC the RX/TX is open for use, though strictly IMO it's better to leave the USB open to connect to the PC for debugging. I have a hub and have run dual duinos at the same time, even loaded code into both as part of the debug cycle. It takes a bit of keeping track though.
That means you don't have a 3-way at the salve and do have special conditions to echo BT through USB though any kind of back and forth could leave you with unexplained results.
I would go with separate channels just to be able to isolate and troubleshoot. At this point I don't see clearly where the bit corruption is coming from and wouldn't expect it. For me it'd be bug-hammer and tongs time. Somewhere there's a lesson to be pounded out, for sure!