hetfield
As you know there is only one pb sw for the forward and one for the aft cargo compartment. So if you pressed the aft cargo extinguisher pb sw as you have stated, the system will discharge both bottles into the associated compartment. If one of the bottles, in this case bottle one fails to fire, bottle two should still fire. The SDCU senses you have fired the aft cargo fire agents but bottle one has malfunctioned and hasn’t fired. The associated BTL 1 light would then fail to illuminate. The system is now considered INOP for the other cargo compartment despite agent still being in bottle one. In this case the forward.
While I can see where you are coming from regarding the MEL, you have just got to interpret it by what it is not saying, if you know what I mean?