Being generous on portions at 1/3 of a lb. per person everybody would probably get some brisket, but pork might be a little short. Figure you are going to loose approx. 40% of the weight once it is all cooked, so if you start with 40 lbs. you will end up with about 24 lbs. of finished product. If the crowd is are big eaters and take approx. 1/3 lb. each, you are looking at approx. 26.5 lbs. each of beef and pork needed.
If you wanted to play it safe do one or two more pork butts, that will give you some overflow capacity. A lot will depend on the make-up of the crowd, if it's high class adults you will end up probably closer to 1/4 lb. per person, blue collar family folk w/ kids will be closer to 1/3 lb. per person.