Now returning to the topic - holding a two-hander in one hand is possible sure, but either fighting or blocking with it this way isn't. So, the only sensibly way to implement it is to make the player unable to fight while they cast a spell. Which would be functionally the same as switching between different load-outs.
Not really. Think about what your character is actually doing in game. You are in the middle of a pitched battle with a companion by your side, swinging your 2H hammer around. You want to throw a heal out on him. "Woah, everyone, time out! Gimme a sec." But your hammer away/lay it out on the ground. Equip your spell and 1H sword. Cast your heal. Unequip your 1H sword and spell. Re-equip your 2H hammer. "Okay folks, continue....."
Thematically it doesn't make sense. It completely breaks the consistency of "the game world", no one would go through all that just to cast a spell. And it adds a lot of needless meddling with extra menus and interfaces. This is an all around bad system that no one from what I have seen has shown any real positives to aside from "its better cause I said so".