In another thread, you mentioned that you were using a SIP phone at home. Usually, that would result in better quality and fewer issues with DTMF, echo, etc. than using an ATA. It is probably possible to set up the office OBi to fork GV calls to your home IP phone, so you could answer from either, and to relay outbound calls so you could make GV calls from either. You can probably do what you want for the SIP provider, too. However, a lot depends on the details.
What is the make and model of your SIP phone?
Who is your SIP provider? Do you have any DIDs with them?
If you do get a second OBi, it should be easy to set them up as master/slave, i.e. the master would register to GV and your SIP provider, forking incoming calls to its Phone port and the slave, and relaying outbound calls from the slave to the appropriate provider.