I no longer flip or mop, just put that baby on and let her go. Remember, your bottom grate is going to run about 10 degrees cooler than your top (generally speaking). Why not put them both on the top? Otherwise you're probably going to have one butt done a couple of hours prior to the bottom one. Or, you could rotate them . . . if it was me I'd just do them both on the top grate, much easier. You're looking at between 15 and 20+ hours. I've had large butts like this go 23 hours . . . they're done when they're done.