Many of my groups have had aversions to playing clerics, mostly for mechanical reasons (my groups consider them "boring" in combats since their role is perceived to be pure back-up and support.) As a result, the GM always ends up making a cleric NPC and then... not wanting to run the cleric in combats, either. So an NPC cleric ends up getting passed around between the players best able to handle the complexity of a support spell-caster.
We found it works just fine, with a few minor issues to be aware of:
1) Make sure the players aren't using the NPCs further their own agendas too much. E.g., in your situation, there is a PC-PC rivalry going on; I would keep an eye out to make sure that this isn't manifesting in the actions of the NPCs.
(If it does, that's a sign that maybe your players each have two PCs each. Which is also okay and can be handled if everyone agrees, but that's not the topic of this question.)
2) For non-support class characters, feel free to give high-level direction for those NPCs. If one of them has an intense plot-driven hatred of wolves, feel free to remind the controlling player to give preferential attacks to wolves when they occur. Doing this regularly both reinforces their status as NPCs, and their individuality as NPCs.
This can, and when appropriate should, include you doing the role-playing (i.e., dictating their speech, if any) even during combats.
3) Feel free to just veto actions that are egregiously out of bounds and which benefit the PCs disproportionately more than the NPCs.
4) If it doesn't work out, just stop doing it. Make clear up front that this is an experiment. If there are critical fight scenes where those NPCs take special roles, you might need to temporarily take them back, too.
But all of those caveats are given out of an excess of caution, or a sense of fine-tuning to increase your overall use of it. We basically had no problems with it. I was under the impression that such arrangements are relatively common.