Hi Orri

Catalyst only support ONE graphics card in a MacPro. You should remove any extra graphics cards if installed.

Theres a few ways of doing what you want to do:
Depending what information you need to see - you may want to consider the network capabilities built in to catalyst.
Either:
Using the internal webserver to display information and give access to preset/cue control.
or
With another server, use 'remote broadcast'. Set the server with projectors to 'broadcast' and set another server to remotely prewiew the first. You should have the same content on both servers in this scenario.
If you want to control the server remotely - then swap this around.

Otherwise:
If you only have one server - You will have to use a Matrox 'Dual Head-2-Go' or 'Triple Head-2-Go' on one output of your graphics card, and connect the projectors to the DH2G/TH2G - this will leave the other output for your desktop/layers view.

Do call if you require further support

Simon