I don’t think you’d find a demon summoning a necromancer very often.
Necromancy is a bastardization of a lot of other forms of magic. But in particular they do have to know summoning/conjuring spells to call the dead back from beyond. All the summoning and conjuring stuff is related, whether you are summoning the dead, a demon, a djinn or bunnies from the forest.
So, pretty sure there are necromancers who can summon demons. Almost positive.
And…well, on this other plane, there was that one Necromancer (who used to be here) that was playing around with a Balrog…but that was in a cavern and I’m not sure it was the Necromancer that put the Balrog in the cavern to try and stop the old wizard.
It did scare the wizard pretty bad, turned his hair from grey to white. And his clothes too. Now that’s what I call a scary demon!