These are the steps to getting 2 demons as a warlock:
1. Get to Felwood
2. Enslave the Jadefire Trickster
3. Have the Trickster attack another Demon (keep the agro)
4. After a while, the trickster will enslave the demon you are attacking
The only way to lose the Demons (other then failing the enslave) is for a loading screen to pop up. (Boat, logging out).
Have fun...
I searched for this, couldnt find it, but if it already posted, /sorry
(I got this off of Ultimate Gold Guide)