Plugging one OBi device into the other has nothing to do with how calls are routed over the network.
There are several different approaches you could take to either set up digit maps to route calls from one OBi thing to the other, or, depending on the ITSP (Internet Telephone Service Provider) you are using, you could create extensions on your ITSP account. Each extension becomes a different SIP user ID/password combination, and can independently register on whatever SIP devices you want to use. Callcentric, for example makes this easy. You start with an inbound DID number, then set up extensions, and make call treatment rules to route inbound calls from each DID to whichever extension(s) you wish, either simultaneously, or sequentially. Anveo has similar capabilities.
A third approach would be to set up a PBX and define extensions there, and then register all the devices to the PBX.