WOW, now that's a load! I always plan 2hrs/lb for boston butts, so that puts you in the 14 hour range for the first leg. If are following this immediately with chicken/ribs, then that's another 3+ hours for chicken and 5+ for ribs. Assuming you were doing the chicken and ribs together, you still need at least 5 hours, which would put you are a possible total time needed of 19 hours (14 for the first leg, 5 for the second). I think you can get'er done in 20 hours, but you still might want to allow extra time to get the butts/brisket done earlier, re-load the smoker with wood (perhaps allowing so cool down time), and then re-load with the chicken/ribs.
Good luck...sounds like a lot of good eats! Cheers