I would say, first fill a can with 8-10 pennies tape top shut. Toss bully and yorkie into the back yard or other place away from all people. Watch closely through a window or whatever. There may be some pushy behavior but they Probably work it out on there own. If you see things getting out of hand toss the can in their direction separate them and try again in a little while. We used to babysit Hank, Mimi the wicked bitch of the west, would get overexcited and hank, the I think I am the dominant testosterone filled bulldog, would get a little too pushy. As soon as Hank's parents would leave, I would toss them out back and close the door. Without us to mess things up, they would figure it out. Usually figuring it out was Mimi rolling Hank on his back. After that, no problems. We didn't have or know about the can thing at the time but I would recommend having one available.
hank is the white and gray dog with the black collar.