The easy way is to have separate GV accounts.
I think it is possible to have one OBi ring the other when a call comes in, and to pass calls through the first OBi for going out. More work, and more to go wrong. That may be OK with you because you can have both ends ring I think. I don't know how to implement that offhand.
There are people who will have 2 locations set to the same GV account not in use at the same time, and disable one and enable the other with OBiTalk.