It's been a while since I did cer so here's my 2cents
2 cer servers is for redundancy so if primary cer fails the 2nd takes over
Cer can support 1 CM cluster or multiple CM clusters
Callflow:
ipphone > 911 CTI RP > CER JTAPI/DB > CM 911 RP to PSAP
Note: CER DB modifies callerid to match the ERL/ELIN. You have to provide the ERL/ELIN file to the local PSAP in the ALI format that the local psap will accept. Check out some common ali formats
http://www.cisco.com/univercd/cc/td/doc/product/voice/respond/ali_1_1/index.htm
so it depends on your local psap which formats they want
so if
site 1 calls 911, the local psap sees the site 1 ali/erl/elin
if site 2 calls 911, the local psap sees site 2 ali/erl/elin
This is because each phone is associated with its a erl/elin that is assign for the specific site in the local Ali format required.