Why, when I use a CV to control both the Rise and Fall parameters (V/oct input), it works oppositely than when I use it to control any of the two parameters individually?

Even though both inputs can perform the same task, i.e., modifying the length of the rising and falling segments of the generated function, their concept is the opposite.

Rise and Fall CV inputs are designed to control the duration of the individual segments so that higher voltages provide more extended envelopes or LFOs, while the V/oct input is used mainly to control the pitch of a generator that is oscillating at audio rate: in this latter scenario, higher voltages equal to higher notes, i.e., shorter cycles. This is why they behave oppositely.