I likewise agree. In particular, a user who needs two GV accounts is presently denied access to 911.
A workaround for some situations: Configure 1st GV account on SP1. Configure SP2 for Callcentric and get 911 from them. Get a free IPKall DID and point it to your Callcentric account. Point 2nd GV account to the IPKall number. You can now add other providers or SIP URIs as gateways.
The principal shortcoming is that all outbound free calls must be sent via SP1, so those pertaining to the activity on the 2nd GV account will have the wrong outbound caller ID.