Yes, it is possible to duo dagganoth kings. Theres a guide in the Player Made Guides that tell you how to specifically do that, but you want to range instead of meleeing. The concept for your style is the mage would get prime on him and use range prayer at all times. Then he would lure rex in a safespot and kill him. Then the ranger would just focus on supreme with mage prayer. Supreme: weak to range, uses mage Prime: weak to melee, uses range Rex: weak to mage, uses melee Rest, you probably know the drill. Get some sara brews, kill the lvl 76s for hp with guthan, blah blah blah.