Setting up multiple GPUs

I am very new to setting up multiple GPUs on my machine. I have an RTX 3070 ti and 2070 super. I am facing some issues and I was wondering if I could get some answers:

1. I can't connect my monitor to 3070 ti. It only works when I connect the HDMI to 2070 super. I tried following these steps but changing device0 to device1 doesn't help. Moreover, is there a way I can toggle between the two device's HDMI without changing the file?

2. I am guessing that because of this, I can't benchmark them together. I am using Unigine Heaven and it only detects 2070 super, unless I don't disconnect the GPU from the power supply, then it detects 3070 and runs benchmark on it.

DO let me know what's going on. I am planning to use them for ML and I don't want to be stuck with running the code on 1 GPU.

Kindly let me know.