Installing OS in a 5.1 Mac Pro

[Gone To Lunch]

The Mac Pro 5.1 has four drive bays.

I have an OS loaded on the first one, and also on the 2nd one.

Can an OS be loaded on to the other two ?

[fruitcake]

Sure. You could load an OS on an external drive too if you wanted. All can be different versions. Say, if you’re not sure which version you want to use. I’ve gotten too consumed with other things, but I used to clone my startup drive to an external drive. That way if the drive failed I wouldn’t have to go through a restore from a backup. Just change the startup drive on launch.
Just hold down the option key when you boot. That will bring up a screen with all the possible boot drive options. You can select which one you want from there. To select which drive you want to be the default startup, go to system preferences, select “startup” and pick your drive.
The thing I don’t think you can do is have more than one OS on a single partition of a drive. You used to be able to do that by renaming one system folder, but I believe Apple doesn’t let you do that anymore.

CAVEAT: I haven’t upgraded to Mojave, that could have changed everything!!