Adding *xx to the phone's dial plan could be redundant in this case. By default, the SPA9000 will accept vertical "star" codes from any of it's registered users. In order to send vertical codes out to the other PBX, you will need to modify the Call Routing Rule under the SIP tab. Using the following syntax, this will allow the codes to exit the trunk that the other PBX is registered on.
Default Call Routing Rule:
If the other PBX is registered on Trunk 2 aka Line 2, then it would look something like this:
You will need to update the outgoing dial plan on that trunk as well. (Line 2 tab in this example)
One last note, if you would like to send select vertical codes to the other PBX, that the SPA9000 utilizes as well, just remember to remove them from the Regional tab. (The previous steps on modifying the CRR will still need to be done)