How do I add another MVP800 to our local PBX?

You may need to order more analog ports for your PBX, and configure them for FXS, FXO or E&M. You will also need a valid IP Address for the additional MultiVOIP on the local network. On your master VOIP, set up the additional ports to be in a hunt group, allowing the additional voice traffic to overflow into the second VOIP. This will allow calls coming into the VOIP from the IP Network to access a free port on the PBX. For hunting to occur from the PBX side to the additional 8 ports would be a function of the PBX.