Step 1: Get yourself a demon enslaved, there are many around the world (felwood is a good place to practice).
Step 2: Duel a friendly priest. You may want to turn off any of the pet's autocast skills for this... start the duel and do nothing, let the priest mindcontrol you and then get you to melee attack your own pet. No spells, just melee! Once you've got in a few whacks, get the priest to end mind control and then run out of range to end the duel.
Step 3: Wait for enslave to drop off your demon. Once enslave drops off you will hear a 'beep beep beep' that is very out of place, trust me when you hear it you'll recognise it.
Step 4: Profit!
The demon is yours without the need to re-enslave, it will follow you even while on gryphons. Unfortunately it doesnt survive zoning, so you wont be able to take the demon from one continent to another or into an instance.
(optional) Step 5: Log out and back in again quickly, the pet control bar will have disappeared but the demon will still be yours to control. You can now summon *another* pet giving you dual pet goodness!
Obviously this mainly hinges on getting a priest you trust to help you out, although it is possible to do this with Jadefire Tricksters in Felwood as they have a mind control debuff that they can use on you. It's difficult and a pain in the ass to do tho... get a priest to help you