If 2 buses are requested and the vehicle owner does not see a particular bus # on the list and knows it is available you should try the following:

Look first to see if a lift has been requested. Most often this is the issue because the list only displays vehicles that are available that meet the criteria of the trip request. If you choose YES you need a lift then you only see buses with lifts.  If the first bus needed a lift,  you can first assign the bus with a lift and then change the setting to No.  This will allow you to select the next vehicle that does not need a lift.

If this is not the issue go to the calendar for that date and see if that particular vehicle is in use and the owner does not realize it. Also look at the GAP time, this may be causing the vehicle to not show.  The Gap time can be found on the Configuration tab, Other Setup Decisions.