Depends on the card, and I don’t have the tools to tell you.
Some cards don’t pull much from the slot, some expect the full 75 watts. I’m not sure if cards can load balance or anything like that 
You’ll just have to try it and tell us if it works.
As for your comment about a waterblock for the mainboard, that’s funny timing because I just posted this other forum post because I’m planning to make one.
And honestly based on my amateur-ish research the easiest solution for a waterblock appears to be buy a vapor chamber and sandwich that between your waterblock and the chip/ram. But the stock cooler also cools the VRM and their public CAD file of the mainboard doesn’t even include the VRM location so until they update the file or I get one I have no idea what my plan will be…
To be clear, I will be CNC-ing something for my waterblock solution, my plan at the moment is to use an existing baseplate from another block and CNC a different acrylic piece, because my CNC can’t do the super fine channels required for a baseplate.