The 911 out and Callcentric SIP in would be associated with the phone connected to the OBi where Callcentric resides. They could also be extended to the other OBi via the OBiTALK Service, but it's been so unreliable for more than a week now with no response from Obihai, I wouldn't want to rely on it for 911.
You don't need the PAP2, but it can be utilized for additional extensions off of an OBi if desired.