It should be possible, and imo the PTR branch should be a separate download just so people instantly know “when there’s new bugs to find.” (This applies to Unturned II and Unturned (version 3).)
However, Classic and Antique don’t really have a reason to be separated. Games that come with multiple things in the Steam library tend to annoy people, and most people will never touch Classic or Antique to begin with. It’s inconvenient, sure, but making them separate doesn’t have much value unfortunately.