No output on second GPU. Precision T7500

I have a Precision T7500 with dual Xeon X5690, 48GB RAM, Quadro P1000 GPU, running Ubuntu MATE 18.04 and Nvidia driver 440.59

I want to add more monitors, so I installed a Quadro K1200 that I had on hand in the second x16 slot. Everything is working fine, but monitors connected to the K1200 don't work and don't show up in the Displays control panel.

Swapping monitors back and forth between the two cards, they consistently work with the P1000 but not the K1200, so I know the cables and monitors are OK. Neither card has a 6-pin connector, so power consumption shouldn't be a problem.

If I run "lspci" both GPU's show up in the list, so the OS does acknowledge that the K1200 is there.

To be clear - my goal is to add more monitors to the system only. I don't need SLI or NVLink or anything like that. Just need both cards to function in the basic sense.

What should I try next? Do the cards need to match? Is there some setting I need to turn on in order to enable the second card?

Thanks for your help!