Follow-up: A better solution for about the same price, plus the USB solution has been a bit flaky with the network manager getting confused about what was connected to what, and a standard ethernet interface causing the ethernet over usb link to drop on occasions. I've ripped the USB link out and put in two new 2.5Gb/s network cards.
I grabbed two of these: https://www.amazon.com/dp/B07RF4SZ8R?re ... E_304_dt_1
2.5 Gigabit Ethernet PCI Express PCI-E Network Controller Card and a cat6 cable. I'm now able to transfer large amounts of data between two machines at 285MiB/s SMOKIN!
There are a few caveats though. First, the card uses the r8125 chipset and the Realtek linux drivers only work up to Ubuntu kernel 4.15, for now. In order to install the driver with the least amount of pain, I added the System76 ppa - it has the r8125-dkms driver. Once the driver was installed, I deleted the ppa. Whatever you do, don't update your system while connected to this ppa, and do
sudo apt update after
removing it. The cable must be standard cat6 to link two NICs together without a purpose-built crossover cable.
So, plug the NICs into your machines and run a cat6 cable between them. Fire up the network manager and add the NICs with a custom IP address. Any IP address will do but to save any grief, you should use a different net, for example, if your network uses 192.168.1.x then set the 2.5Gb/s card's IP to, say, 192.168.10.x, and if you use 192.168.20.x, set the IP to 192.168.2.x. This prevents your network getting confused about where to route your traffic.