Sorry that you can't use two WS versions. You can either use WS pro 12.5.7 or WS pro 15.5 at the same host.
Shazzbot. I thought as much. Thanks!
Caveat - as long as you don't modify your guests while running the v15 trial (i.e. don't upgrade the virtual hardware or VMware Tools), you can still safely uninstall the v15 trial and re-install (and re-license) your v12 Workstation.
> You can either use WS pro 12.5.7 or WS pro 15.5 at the same host.
That is not really the complete story - you can actually use both versions side by side if a reboot is an option.
But setting it up is not trivial