The big question is are you splitting the meat or serving a portion of each?
Personally I'd offer a serving of each / plate, so 60 x 4 = 240 / 16 = 15 lbs x 2 = 30 lbs pork butts, the same figuring on the brisket is around 25 lbs, this should give you enough for 4 Oz of each meat / plate. These figures are tight for an accurate count of 60, if you really have more guests, you'll be short on meat, if the count is lower there'll be leftovers.
"There is no such thing as a little garlic." A.Baer