Since I believe DC is aware of this bug I will not go into the details. However, one way to prevent the duel-hero glitch is to prevent the load feature from being activeated immediately. You could do this by putting a 120 second delay on the load; this is usually long enough to eliminate all the late loaders. I rarely see one that lasts longer than 2 minutes. Also, even without loading you can still do if you are fast enough. Another option is put in a delay feature that changes the player's name to something else like "Unknown" until after the cutscene loads and the whisps are spawned. This would also prevent the glitch abuse. Also, I am not certain if you were aware, although a player can also transfer items directly from one to the other. I.e. a 999 can powerlevel himself then give all legendaries to the other hero in the same game. If nothing else this should be eliminated.
Thoughts? (Nobody say the glitch on here directly please... Rather not see it happening anymore than it does... I think DC knows anyways.)