Must be pretty confusing indeed cause you lost me on that last part. If you cab has 2 x 8 Ohm or 2 x 4 Ohm inputs ( one left, one right ) then you can use each half of the power amp for each half of the cab.
Example:
2:Ninety
Output Channel
A Speaker out @ 4 Ohm Into Cab Left @ 4 Ohm
B...