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.