That's such a cute story racheldarcy! You are full of delightful ideas that I am sure you won't have trouble thinking of new ones to get those boys potty-trained in no time.

But since you asked, I really personally think that you should persevere with Toby and not give up especially since Sam is definitely showing some signs of progress. It would be unfair to Sam (though he may not realize it at all) to have learned to use the loo and risk loosing all of that progress.
If you stop now, you'll have to go back to square one with the two boys, and I don't think that's such a good idea.