You can learn a new mode of thought. You have to be open, and not dismiss things early, which means allowing yourself to follow ideas which have obvious flaws or which totally impractical. It helps to have confidence that you'll use your practical, analytical, and more closed mode of thought later. But first you've gotta jump out of the local maxima and try to find something new. Generate ideas as if you're high; highdeas are usually bad and that's ok, your sober self will be around to check it out later. Maybe the idea will be thrown away then, and maybe it won't.
There's a great John Cleese talk on open vs closed thinking that you could look up on YouTube. A good book on the subject is A Whack On The Side Of The Head.
Edit: That said, there's also something to working with another person, and each focusing on one of these roles more than the other. Like author and editor. Lots of things follow this pattern of splitting these roles up among people.