I think you can do what you want with CallCentric, by using extensions.
In the Callcentric dashboard, you'd first set up an extension (e.g., 101), then go back to the dashboard and in the call treatments you would set up a rule for simultaneous ring of both extensions.
Register the first OBI as normal with your SIP user ID of <1777xxxxxxx> and password. Register the second OBI the same way, but using the SIP username of <1777xxxxxxx101> in the OBI-- basically you append the extension number to the SIP username...
Callcentric offers free NY DIDs, so you can get a free DID which you could have GV forward to that will ring both OBIs. If you don't want to use Callcentric for outgoing, then you'll need an outgoing provider on the other SP of the OBIs. My current choice for that is Circlenet, since it's pure pay per use at very good rates. I am a light user (this is a secondary line -- we have FiOS phone as a primary), so I would rather pay a few bucks a month for my actual usage than pay a monthly fee for a bucket of minutes.
As a disclaimer, I assume this works for outgoing too, but I only use Callcentric for incoming so I can't be sure.