Long ago, they'd decided to let Edward have control out of simple courtesy; it was his body, after all, and he knew how best to act. These days the only time the other Ed came out was when he needed to talk with Hohenheim; such strange conversations they had, of alchemy and alien worlds and gates.
Other than that, Ed had full reign over Edward's dreams, because while Edward was sleeping, Ed had nothing to do but think. Both of them enlosed in the recesses of one mind, they each had access to the others' thoughts.
What's that? Edward asked, referring to a mushroom cloud.
I'm not too sure, I only saw it for an instant. It's something from your world, though. Ed explained.
I've never seen anything like that. Are you sure? It looked too...foreign and unnatural, like their talks of alchemy.
It's not like any alchemy I've seen before, either.
And that? A room glowing with a bright scarlet liquid loomed up in the mind-space.
Oh, that's from mine. It's...just don't ask.
Obviously important enough for you to think about, though.
Shut up! Mind your own business!
And how do you suppose I do that? Stop thinking, I'm trying to sleep.
Yeah, like I want to see your dreams. At least mine are somewhat interesting.
I like my dreams, thank you very much! Now bugger off!
When Edward woke up, he felt like he hadn't slept at all.