Nexus Clash

60cp child of Eldritch Mixing, makes it so both spell affinities are applied when mixing in a second damage type per Eldritch Mixing. You've invested 160cp for both spell trees, plus 90cp for this new skill and the parent skill, plus a few others in the Wizard list to really make it shine (in particular, Fortify Spell and Arcane Affinity), so I'd say you should get to reap the benefits. It's comparable in power to (though still a touch weaker than) an Archon's fully upgraded Word of Wrath.

Spell damage bonuses:
Wizard: 4 (60 cp)
Corruptor: 4-6 (90 cp) depending on whether you get draining effects. Notable utility compared to other classes, both in terms of mp efficiency and buff removal.
Archon: +13-16 (150 cp? But 90 of that only gives +2 and also gives skills you probably want anyway) including fully stacked wrath and divine vengeance, another 2-5 from stacked prayers that can be gotten for cheap from sanctify spell. Good utility from other words if you can afford the mp. Notably mp limited without advocate support.
Elementalist: +5 (180 cp, but you'll be spending it anyway)
HC: +11-14 (150 cp?)
DH: +~3 (no cp lol)
NC: not enough mp anyway
Others: meh, just prayer/Lord over domain/minor bonuses

I think mixing/spell wizard needs a little boost to be comparable. But 5 is more than I'd give. If the ability is boosted to give both capstones without actually maxing both trees, that would be a great start. Each capstone is worth about 2 damage.

Also noting that this would make mixing+child +a whole second spell tree (+170 cp, total 230) pretty much mandatory for spell wiz, rather than a utility bonus as it currently is, meaning a lot of cp on getting to basic damage cap and creating quite a big power differential between basic wiz and rank wiz.

I think it'd be better to just make this an effect of Eldritch Mixing, and increase the cost to 60 CP. Dual-element Wizards are already CP-strapped as-is. It'd allow more flexibility when leveling up as well, since they can take Mixing early and get a slight damage bonus before getting the capstone for both elements.