I too am a little confused. In the past I have ordered more than 2 aircraft on the first day, with one order being for used aircraft and the second for new aircraft.
I have posted 2 screenshots:
The first I understand, as order 1 is the used aircraft in service and order 2 is for 2 new aircraft.
The second I do not understand, as order 1 is the aircraft in service, but order 2 is for 2 different fleet types.