I know two heads are better than one. Does that apply to writing projects?

So there I was driving along in my car on the way home from a family outing, my husband asleep in the passenger seat, my kids snoozing in the back.  The music was low.  I was twirling my hair not thinking about much of anything, letting my thoughts go where they would then KABLAM! An idea for a book.

And there I was with nothing to write on, nothing to record with, hoping I could hold onto the contents of my brainpan through the rest of drive and the long process of bedtime.  I clung on with all my strength, conserved thought energy by responding to my husband and children with noncommital grunts then dashed to my laptop the moment the last goodnight kiss had been given.

I pounded out my ideas and thought 'Okay, well that's that.  Now I know what I'm going to be working on when my WIP is done."  Or so I thought.  That idea rattled and tapped around in my head, tantalized me with its newness, its potential, its mystery until I finally said, 'I give!' So now I'm working on two projects; one revision, one first draft.  It's the first time I've ever done this and I'm a little nervous.  My main concern is that the projects might bleed into one another and they are so very different I don't want one to compromise the other.  For those of you who have or are right now working on dual projects, how do you prevent crossover?  And do you give both projects equal time or do you let your muse dictate which one you work on and when?  Any advice would be much appreciated.