So, based on 13th age monthly's Summoning Spells rules, Wizards can only order around one 'ordinary' summoned creature at a time, but in exchange, they can have multiple summoned at once. Necromancers, of course, still use their rule that they can only have one summons at a time.
But... what rule wins out if a Necromancer takes the Dead Wizard feat, and takes a wizard-based summoning spell for their spell? The way I look at it, there's 3 possibilities:
1) The Necromancer specifically has Summoning as a class feature, and so ALL summoned creatures for him use the default 13 true ways rules, treating them all as 'superior'. This means no actions spent on them, but it also means that you can still only have one at a time, no matter where the spell comes from.
2) The Dead Wizard class features treats a wizard spell fully as a wizard spell, following its own separate rules, so you get the best of both worlds, or close to it; you can use only a single Necromancer summons as normal, but may then also use your Dead Wizard spell(s) to summon ordinary creatures on top of that, since they may be summoned even when you have other creatures. But this allows for things like spending standard actions ordering Ordinary summons around while your Superior necromancer summons work on their own.
3) The Dead Wizard class features treats a wizard spell fully as a wizard spell, but this doesn't override your Necromancer limitations, so you get almost the worst of both worlds; if you use a Wizard summoning spell when a necromancer summons is present, or visa versa, the stronger necromancer one disappears.
I'm currently leaning towards the second one, if it were up to me, because to get to this point you're choosing a specific talent, you're taking the feat for that talent, and you're using that feat to gain an obscure summoning spell instead of likely more potent options elsewhere on the Wizard list, and you're likely thus already focusing hard on summons, so it makes sense for you to eek out some extra benefits.
This question could also apply to, say a bard using the champion feat of jack of spells to get both a necromancer/druid and a wizard/cleric summoning spell, though in that case I'd think it's between the latter two options.
EDIT: I suppose the question also applies for multiclassing in general.