Don't do it, turn back, save yourself, it's not too late...okay
Well, you have to look at the overall meal, how many sides etc...fewer sides needs more meat etc...I would look at the fact that there are two meats, and figure 1/3 pound total cooked meat serving per person in a normal meal situation. This allows for men who eat more and women who eat less. If it is all men, go 1/2 pound cooked meat per person (shave the side portions, as men tend to eat meat and don't eat as much of the sides).
Tri-tip cooked yield loss is about 15% of weight to medium rare, 20% for medium.
Chicken is around 30% of weight to proper temperature.
Assume 50 adults by 1/3 pound serving, you need 17 pounds of meat total. Split the difference, you end up with 8.5 pounds of meat of each type.
8.5 pounds x 1.20=10 pounds of tr-tip raw
8.5 pounds x 1.30=11 pounds raw boneless chicken
Thus you need to buy at least 10 pounds of trimmed tri-tip and 11 pounds of boneless chicken. This would be for full sides or sandwiches. If you are doing a meat and two meal, with say beans and slaw, tack on another 4 to 5 pounds to the meat order.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."