Premiere will not use more than one card. FCPX will (you can always look at GPU temps or GPU activity in Activity Monitor to double check).
As far as recommending it, I can't because I haven't tried it, so this is all in theory.
It's such a niche card it's going to be difficult to find an editor that is going to buy dual Radeon VIIs and give performance benchmarks.
There are benchmarks on the Windows side for 2x Radeon VII's, though.
See if this helps with your decision:
The Radeon VII is a very interesting card for DaVinci Resolve due to its 16GB of VRAM which is twice what you would get from a similarly priced NVIDIA GPU. But is it able to keep up in terms of raw performance?
www.pugetsystems.com
Also for Premiere with a single GPU (this is useless for you since you have 1 gpu already):
Depending on the number of GPU-accelerated effects you use, a higher-end GPU can give you a nice performance boost in Premiere Pro. But what will give you the best performance for your dollar? An NVIDIA GeForce RTX video card, or one of AMD's Radeon cards?
www.pugetsystems.com