I did 2 flats and 2 butts in mine. I put the flats on the bottom, fat cap down, and no trimming. Since I trim my fat cap off of the butts, it really didn't matter which way they went in. Takes longer cook time on the butts,though, so I started them early.
Don't ask how I know this...but if you FTC, you might allow for carry over cooking.
Oh!, might watch the drip pan...LOL!
Good luck!