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 2 Yellow Bus and in the comments, they indicated they need a second Activity Bus.   

Assign the first vehicle which will be a yellow bus.

Scroll back up to "Type of vehicles needed to reserve" and change it to Activity Bus.  

Choose the second vehicle and reserve the vehicle.