This might be a situation where you need one Yellow bus and one Activity bus or 1 Yellow bus with a lift and 1 Yellow bus without a lift. Here are the steps to make this work:
Scroll to the Reserve Vehicle Section and reserve the first bus: (This example needs a 2 Yellow Buses. In the comments, they indicated one needs a lift and one does not need a lift.
Since Need Lift = Yes we will assign this vehicle first. Choose the vehicle and "Reserve Selected Vehicles"
Scroll back up to Need Lift= Yes and change it to "No". Choose the second vehicle and "Reserve Selected Vehicles".
Choose the second vehicle with no lift and reserve the vehicle.