I'm (sadly) almost certain of the answer to this question already but I wanted to make a topic so that maybe some of you might say something to the contrary.

I have it in my imagination that while dual wielding with two melee weapons you're only able to block with one and attack with the other. In other words: You have two buttons (press button A and you'll attack with one hand/ Press button B and you'll block with the other).

I was sorta kinda maybe hoping that you could have more freedom and choose which hand to attack with/ block with and whether you want to attack with both hands or block with both or alternate between them. I'm only leaning to my initial theory because I can't see how the second theory could be implemented on the PC (you would need four buttons on your mouse or something)

I don't think you can block while dual wielding but I don't think the implementation would have needed to be as hard as you're making it sound. Blocking is timing sensitive in Skyrim so they could have just made it where an attack command given at the right time would have resulted in a parry. That said, I trust Beth, so I'm sure it's all balanced and/or viable.

There's no blocking at all with two weapons. Done, as far as we can tell, for game balance. However, it seems pretty likely that you can parry. If you time your swing with an enemies, your weapons will clash mid-swing, you take no damage, and are both very briefly staggered.