the motherboard black and blue x 16 slot (physical x 16 both and electrical x16 + x4) can support two weak gpu at same time, I think.  to prove this you may use two low power gpu such as HD6350 (20w). but to support 1060 + 1050 card with TDP of 120w and 75 w is taxing the mobo way over board. although the tech guide says 35w max for pciex16 slot, it is known that a single 75w card such as 1050 is supported.  this is already above-the-spec use.  when you use 1060 w 6 pin the power issue is ameliorated by the 6 pin supplementary power.  I suppose to prove the concept, you may also try two gpu both w supplementary 6 pin in two x 16 slot to see.  but playing with mobo that has weak power upper limit in PCIe slot is risky. also we do not know whether the 35w max in x 16 slot is determined by Dell stock psu rating, or actual upper limit of power rating for the slot.  a standard x 16 slot is rated to support up to 75w.
Hello - hoping to get some insight.  I am running the following: MB: i5-2400 @ 3.1GHz / RAM: 16GB /  PSU Pystar 1000W (80 plus gold) Windows 10 Pro (v. 20H2) / BIOS: A22 installed and working GPU: Nvidia 1060 3 GB installed on PCI-e slot 1. Issue:  I tried installing a second GPU Nvidia 1050 on PCI-e slot 2 or 4, but the PC will not boot. If I swap them around, the 1050 works by itself, but not combined together. I tried using a riser, same issue. I am using Nvidia driver 497.29 With both cards installed the PC will not boot up.  Thanks for any help!    
Optiplex 970 not accepting 2 GPU