Cordyline Australis "Red Star"

As far as I'm aware, Cordylines shouldn't be "cut" or pruned. What exactly do you want to do with it?

They are getting to tall so I would like to cut them in half and propagate the upper half. I was told this can be done ?

I've no idea whether that's true or not, I'm afraid. Even so, this kind of thing, I would have thought, would be better done in the spring, and yours must be approaching now ,or you're already in it, so give it a go now, if you want to try it.