I don’t disagree, but it is Valve’s IPC/Signin process that demands this. I tried several VMs and Sandboxing tools with no luck. If you find you can wall off a second Steam client on the same machine, please let me know.
The few attempts I made looked to work “mostly” until I started getting cross talk between the two clients. IPCs meant for one client showed up on the other and I immediately stopped testing that way.
I wrote a custom batch file using robocopy to sync the two machines. Less than ideal, but as efficient as I could make it.