Hi Marty,
I think that both the Yamaha and B2 USB sockets are 'master' sockets (the B2 USBs definitely are) and therefore should not be connected 'back to back'. You run the risk of damaging the USB sockets electrically as they are both supplying 5 volts output, which is not a good situation.
The only devices that should be connected to these 'master' units are USB 'slave' devices, such as USB HDDs, memory stick, bluetooth dongles, wifi dongles, keyboards, etc.
In any case, the B2s do not supply audio output from the USB sockets.
Regards,
Peter.