Google Chat supports only one 'registration' per account. If two or more devices try to connect, an incoming call will go to the device that sent the last request. AFAIK, there is no way to bypass this restriction.
Some possible solutions:
1. Remove Talkatone and get http://www.obitalk.com/forum/index.php?topic=41.0
-- the OBi will relay GV calls to/from the iPad.
2. Use separate GV accounts for OBi and Talkatone. Of course, the devices will then have different numbers. If e.g. you mostly use one and your wife mostly uses the other, that may turn out to be an advantage.
3. Have GV ring the devices via a SIP service, using the Google Chat path only for outgoing calls.