You are not going to get two bulls to come duke it out from your calling. Each bull has a unique call that they know each other by. They also know the exact location of the other bull at all times. When you call with a bugle, they will come into it because you are a new bull, and as such they do not know if you are a threat to them. However, they won't fight each other because they know that it wasn't the other bull calling, it was you. Since you are using a cow call, bulls will come into you, but they will not fight over a single cow. It is more of a first come first serve snatching. Also, if the herd bull believes that by going after the new "cow" (you) he will lose his other cows to a satellite animal, he will leave you to the other bull. When you see two bulls fighting, they are not fighting over a single cow, but the entire herd.
As far as calling in bulls, unless they are extremely rutty the herd bull will not leave his cows. Once you call a bull into about 100 yards, if you call any more, he will be able to pin point your location and he will circle you until he is downwind. Once he reaches 100 yards, it should be up to you to close down the distance without calling.
Your best bet is to find a herd and the herd bull, and follow it until a satellite bull challenges. However, a physical confrontation is very uncommon, and most of the actual challenges are settled by bugling. The only time when a physical confrontation occurs is when a bull of equal size challenges the herd bull, and neither animal backs down.
Hope this helps! Though this is a little late in the rut for most of this to occur 
This is all from my experience from the past 15 years of hunting the elusive Wapiti.