For either adapter you'd also need to buy a power supply to power the risers/cards. Keep in mind that using Molex connectors/adapters isn't always reliable, and I have seen stories about people melting adapters or even starting fires. The pins on the power header on the riser that comes with the E key adapter in particular don't look like they can carry a lot of current.
With either adapter, you're only going to have PCIe x1 bandwidth for the card though. What card(s) are you planning on using, and what do you intend to use them for?
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.