Dual monitors on my HP Pavilion Slimline 400

I recently purchased an HP Pavilion Slimline 400 with Windows. My main reason for purchasing this particular model was the ability to run dual monitors. I have an HP L1906 and Gateway FPD1730, both are flat-panel monitors. For some reason, it is not recognizing anything plugged into the DVI-D port, only whichever monitor is plugged into the DVI-I port. Please help me. Thank you.