This is a frequent question please have a look here:
You don’t have to take further action. Within Blender you can simply mark which cards it should use to render.
Just so you know 1: You can even mix different models. I just recommend you to choose all to be either NVIDIA or AMD.
If you’d like to hear my opinion I’d get as much from the current NVIDIA cards as you can.
JSYK 2: All of the said is true for Cycles. When using Eevee it will render on the card that is driving the display. Although you can, e
g. for animation, start a headless renderjob for each card and let them work on different frame ranges.
Hope this helps