Use a different Google Account for the second GV phone number.
On my OBi202, I use a VTech DS6151 2-Line Expandable Cordless Phone. I use the VTech Menu Option to configure different Ring Patterns for each of the two phone lines.
In OBiTalk, you have the option to select either Ph1 or Ph2 or both Ph1 and Ph2 to ring for each GV Phone number.
I use:
Ph1 for GV Ph1 calls. Used OBiTalk Expert Mode to configure
Voice Services > SP1 Service > Calling Features >
CallForwardOnBusyEnable [X]
CallForwardOnBusyNumber ph2(#)
Ph2 for GV Ph2 calls. Used OBiTalk Expert Mode to configure
Voice Services > SP2 Service > Calling Features >
CallForwardOnBusyEnable [X]
CallForwardOnBusyNumber ph1(#)