If your monitors DVI input, you need to get minidisplay to DVI adaptor first.
Connect one display to DVI port, the other connect through minidisplay to DVI adaptor.
All Mac Pros come with graphic cards that support at least 2 monitors. The earliest had DVI connectors; more recent cards have mini-Displayport connectors, you may have a combination of both. You either need direct cables (ie, miniDP to DP; DVI to DVI) or adaptors which let you convert the connector.
If you're having a specific problem with it, please let us know what Mac Pro you have, what display connectors you have and what monitors you're trying to connect.
Note that Mac Pros will NOT work with a Thunderbolt display.