Cage training is a really great tool for potty training but it may be too late for that.
If you ask me, one of the best tools you have right now is your bull dog. He's trained and can show the Yorkie how its done. Take them out together!! Plus getting them on the same feeding schedule would also help.
Leash training also called
Umbilical cords is another great way for potty and discipline training. Its a way that you are there every moment with them. You can train them and catch them in the act and put a stop to it quickly with the "aah aah" and a gental tug on the leash. If they are getting ready to do their business... you can stop them in their tracks as well and cart them off to the yard. (take you pup with you for some back up/your pup can show the other what and where to go).
treat the good behavior and put a quick stop to the bad behavior!!!
I have some information about both on my site but you can also do some research about it on line. below is a very informative site:
Training Your Dog
Good Luck...