Hi Doug,
I do not understand what you mean by "saving the USB port for RTTY". The USB port provides four completely separate virtual COM ports (COM0, COM1, COM2, COM3/COMEXTRA) of which using one has nothing to do with the other one. Of course, your K3 configuration means that you can not use COM0 or COM2, but of course you can use COM1 and COMEXTRA at the same time, assuming you enable them in the RRC correctly.
73,
Mitch DJ0QN / K7DX