I work in a company where we own an Arcgis Enterprise standard (ArcGis Server, ArcGis Portal) version 10.5.1
it is working on our virtual windows machine.
Now we want to upgrade to version 10.7.1.
We want to install all the new software on a new vitrual machine, copy everything there and test how everything is working while letting our main server running.
The question is if it is possible from the Licencing point of view to have 2 servers running simultaneously?
If you're implementing disaster recovery using geographic redundancy, you can use the same license for each, as only one site is active:
Additional ArcGIS licenses are not required for the standby deployment because it is not actively accessed; you only make it the active deployment if the primary fails.
You're somewhat in the same situation; you're setting up a new environment that won't be production, and will just be used for upgrading. Eventually, you'll cut over, but as long as you're not configuring both to be active and receiving requests as if they were both production, then you'll be fine.