Two pair hashes compress four leaves into two parents.
Leaves are hashed in pairs
A and B form parent AB. C and D form parent CD.
A plus B wraps to 65
For AB, 0xaa plus 0xbb gives 0x165, so the low byte is 0x65.
H(aa∥bb)=0x65
C plus D wraps to a9
For CD, 0xcc plus 0xdd gives 0x1a9, so the low byte is 0xa9.
H(cc∥dd)=0xa9
Summary
The parent hashes are AB=0x65 and CD=0xa9.
AB=0x65,CD=0xa9