Yeah. Just not when you have a spell in one hand and a sword in the other.
Unless you actually
can do that, by pressing both buttons simultaneously.
I've given it some thought, and I'm asking "why would we ever need to press both buttons simultaneously for the purpose of attacking?" If you dual-wield weapons, the only double-press attack would be a scissor attack of sorts, which isn't really that important to have in a game (plus you could still have this attack happen if you released the buttons immediately after pressing them, to prevent it becoming a block). For combo-wielding, you generally cast the spell
then swing the sword, and so on (not necessary to have the ability to do both simultaneously, because either action happen so fast). Finally, if you two-hand wield something, it should also be enough to just press either button. So that leaves pressing both buttons simultaneously in order to block,
regardless of what you're wielding.
It makes perfect sense.