You can run virtual machines (virtual box, VMPlayer) and use a different steam id in the virtual machine os and another one in host os. Ubuntu is a good choice since it’s a lighter os and so virtual machines should not have performance issues.
As far as i know there is no way to run two steam users in the same os, or have the game run twice with same steam id (except maybe dedicated servers)
I personally use a old laptop with windows 7 and a separate stream id to test steam connections