I've been looking for a few weeks now, I've read a lot of the book and have tried searching online, but I haven't been able to find anything about wielding two two-handed weapons, one in each hand, or even about whether I can wield a single two-handed weapon in one hand. I'd be pretty happy with an answer either way, it's just not really mentioned anywhere that I can see.
It's probably mostly a Black Crusade thing. I doubt the other WH40k games would have the issue come up often, but I have a 12 Strength bonus at the moment (inclusive of unnatural) and I'm thinking it makes sense that I could, but also it might be game-breaking if I did.
Best Answer
It is actually possible for a space marine to dual wield two-handed weapons, though a DM might not allow it.
In BC, space marines are automatically ambidextrous. And, as a space marine, you can take a power armor mod called 'Recoil Suppression' that lets you use Basic weapons one handed without the penalty.1 Slap on the Two Weapon Fighter (Ranged) talent, and you're good to go. Although any shots are going to be at a -10 for both weapons. Not much of a problem if you're fighting hordes, though.
Note that while this works for things like Legion Bolters and stuff, it won't work for any weapon heavier than Basic. So no dual-wielding lascannons, sorry.
A normal human could also do this with pistol-gripped (a weapon upgrade) weapons (say two autoguns) and the Two Weapon Fighter (ranged) talent. And perhaps the Ambidextrous talent, too.
Enjoy your heretical dakka, traitor!
1 Further supporting my assertion, I found this piece of text while crawling the book again (p. 244):
So one needs to be able to reasonably convince their DM that the recoil suppression on SM armor allows one-handing bolters freely. Or simply use smaller bolters.
Not sure how this applies to human characters, unfortunately.