Not a common scenario but I have a Unity Pro perpetual license and the company I work for has a paid subscription license.
Can I install BOTH licenses on the same machine, effectively having 2 licensed versions of Unity on the same machine?
Technically this is against the lisence terms. You can only install a lisence on a device you own. So if it’s a company machine, you can’t install personal. And if it’s a personal machine, you can’t install the company lisence.
In practice there are some weird tricks you can do involving the lisencing file. Or you can return the lisence each time you need to switch.