Yeah, I've seen mPCIE M converters everywhere, but the E slots are different.
From their description:
1x M.2 M Key, PCIe 4x, Supports NVMe SSD and SATA SSD
1x M.2 E Key, PCIe 2x,Supports USB2.0, UART, PCM
The M slot has been tried, and works well with a 2080, and the E slot has not been tested, but it's speed should work well with a 2060, provided there exists an adaptor.
For folding, both slots would work well on 2x 2080 ti with only a 10% drop in performance (perhaps slightly more on the 2x slot).
Best picture I have from the mPCIE slots is this one (the M slot on the bottom (left), the E slot on the top):
Edit: Too bad the PCIE E slot (2x speed) only converts to PCIE 1x. This is unfortunate!
I was hoping to maximize the PCIe lanes.