IMO it sounds like Ty definitely understand when he is going to or is weeing/pooing but maybe still a bit too young to grasp the concept of having to do it on the big persons toilet/ or even a potty?
I have heard on rare occasion of some girls being potty trained early, and it's definitely not a bad idea putting him on the potty to get him used to it but boys do tend to TT later than girls (someone feel free to correct me if I'm wrong here, but I'm sure I've read this somewhere in the past ...?). Might be expecting too much for a 19mo, but definitely not doing him wrong by letting him sit on it and get the gyst of it all iykwim. I can definitely understand with the state of the economy, that you wouldn't want to have 2 kids in nappies if you can help it.
Good luck with whatever you decide though. I have a 22mo and whilst we have a potty here, he has shown no interest in it yet but he does understand when he's going to the toilet in his nap. I have a couple friends who have left TT with their kids until just before/after the 3year mark and their kids have managed to go from wet to dry within just a week!
Let us know how you get on though! Best of luck

(sorry for all the waffling, far too much time on my hands hehe)