OK, I just spent a few minutes on callwithus' website, and I can't figure out if they have the features you need to do this. You probably do, but either way, I'm going to use an example how I'd do it with Callcentric.
Callcentric gives you an 11 digit account number of the form 17771234567. You can create extensions, too. For the sake of argument, let's say you've set these up as 101, 102, and 103. They also have longer names, based on your account number, i.e. 17771234567101, 17771234567102, etc. The short numbers are useful for dialing. The long numbers are needed for their Call Treatment features, I think.
With either of these, you have a choice of using the device to do sequential ring, or using their website to do simultaneous ring.
Sequential ring is accomplished by setting the CallForwardOnNoAnswer fields on the LINE port. Let's say after 4 rings you want it to forward to the Obi200. So in the CallForwardOnNoAnswerNumber field, you'd put SP2(101), which you've cleverly programmed on the Obi200 to receive the calls, probably on SP2.
So an incoming call would ring on the Obi110 for 4 rings and if not answered, would ring on the Obi200 where it could be answered.
Now, to do simultaneous ring, you need to use both SPs on the 110 and one on the 200.
You'd forward LINE to SP1 where it would call 17771234567. On the Callcentric website, you would set up a Call Treatment that said to simultaneously ring extensions 101 and 102 (which you've installed on the Obi110 SP2).
That said, I'm not sure how you handle AutoAttendant with this situation. If AutoAttendant picks up, the Obi110 stops ringing. You'll also need to pass a callerID from the line port. Don't have a 110, so don't know how possible that is.
Anyway, see if any of this translates to CallWithUs.
Good luck.