I think you have that statement backwards.... caller ID is only supported on FXO ports if you are using H323, not MGCP. So it depends on your deployment.

RE:

can two seperate Call Managers register to the same MGCP gateway?

An MGCP will only register to 1 CCM at a time. You can however setup several more call managers as redundant-hosts. The Call Manager you want it to register to will be setup as the call agent. After this is setup, a show mgcp command will show the primary as registered, and the secondary as "backup ready" If the primary fails, the MGCP GW will failover to the backup.

Hope that helps!