As my hardware testing continues, here are the results for 1 video card vs. 2 video cards on identical projects with the same computer. I turned the 2nd video card off within the Pix4D options instead of tearing apart the computer.
Your own projects will vary because the video card gets used differently...this test is from 3D modeling of a cell tower. Hopefully this will help others when trying to determine the best hardware setup.
Single GTX 1080Ti (had to use XML file as Step 2 crashed and erased PDF):
Dual GTX 1080Ti (adds 13% cost to my computer):
The result for this project is a 4.9% time savings in Step 1 from a single video card. Step 2's point cloud creation gives no benefit from a second video card so the overall time savings is even less...all for another $800 or 13% system cost.