Do what everyone else does. Stick your twink in the zone your doing dungeon in. Switch at last boss from 80 to twink at 10%. get xp credit on twink. Lots of people do this, and it’s considered acceptable by most everyone cause it doesn’t gimp the run at all. Still nice and fast. Plus look at it this way, you run your 80 you get 80 loot the entire time, it’s a win win.