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.