if your primary Obi is a 110 you can daisy chain them.
connect the phone port of the 2nd obi to the line port of the 1st obi.
pressing # on the 1st obi will give you the dial tone from the 2nd obi. From there you can do anything as if the phone was plugged into the 2nd obi.
This will allow you to use the services of both obi's from the same phones that are connected to obi #1.
Pick up the phone, press # and you get a 2nd dial tone. Dial a number and it will be delivered using sp1 of the 2nd obi. Same as having a land line connected to obi #1.
This also may remove the need for you to forward the calls to the other google voice number. If you are only forwarding to have the calls delivered to your home, you won't need to forward them anymore. If you are forwarding them so google voice can fork them to the proper destination phones, you can also do that from the obi itself.
With your forwarding setup you may find that calls don't ring long enough on the 2nd google voice number because the call will ring once before the obi forwards it, then ring once again at google voice before it knows what to do with the call. So that leaves about 2 rings before the voicemail for the original call picks up.
If you do not want the 2nd obi to ring the phones you will need to edit the call route for sp1
voice service -> sp1 service -> X_InboundCallRoute
default is "ph" an empty rule {} will do nothing with the call. If forwarding is set the call will be forwarded, if forwarding is not set the call will be delivered (after 25 seconds) to the google voicemail for that number