I was killing one of the Jadefire Trickster mobs when it used its mind control, which only lasts a few seconds.
I noticed once I was out of the MC, that the spellbar for my enslaved demon was gone, I couldn't control it.
It would however still engage whatever I was fighting. I enslaved a new Hellfire caster mob, and to my surprise the other one was still enslaved.
I was able to run around for the next few mins with two functional demons. I didn't test it further, but I'm guessing it might even be possible to have even more enslaved demons this way.