smilem Multiple nvidia cards in the same system use a single video driver and so you need to disable driver signature enforcement and install a modified driver that recognises both cards. The quadro driver has an .inf file in which you have to add the hardware id of the rtx video card(found in device manager). Here is a guide or you can use an utility called NVCleanstall that simplifies the process and also allows driver signing.
I think the Kepler Quadro K4200 + Ampere RTX3090 combination only works with modded .inf drivers between versions 456.38(start of 3090 support) and 475.14(end of kepler support). Fermi support ends before the 3090 launch with driver 391.35.
Other useful information:
https://ledstrain.org/d/1688-rtx-a4000-causes-eyestrain-with-true-10-bit-monitor/48