Dual Monitors On Optiplex 745

We have an Optiplex 745 in our Datacenter upon which I am asked to configure dual monitors.  It has an on-board VGA port and a Radeon X1300 card.  When a monitor is connected to both ports the pc won't even enter BIOS but displays an error telling us to disconnect on of the monitors.  I have changed the Primary Video setting in the BIOS from "Auto" to "On-Board" and back but the error continues.  What must be done in order to get both monitors to function?