Is it possible to install the Fast ring for Windows Insider on one VM, the Slow ring on another, and keep my main machine running the release build of Win10 1809? If so, how, and would I need multiple Microsoft accounts? Thanks!
If so, how, and would I need multiple Microsoft accounts?
Outside of the obvious licensing issues, you would just change the release channel of the VMs, and the same MS account can indeed be used.
The enrollment to the Insider Preview release branch is per machine, virtual or physical, and is not connected to your MS account.
It’s as simple as creating two virtual machines, installing Windows 10 and changing the release channel to the Insider Preview branch, once upgraded to the current Insider Preview build changing the Insider Preview branch on one of the virtual machines.