You don't have to switch it to stereo. That switch splits the amp into two independent pairs of speakers. Think of it as two conjoined 2x12

hm cabs.
When using a JCM2000-series head with two JCM1960-series cabinets (the standard 300w capacity versions), your option is really to switch the amp to

hms and run the amps speaker outs (the ones used for 8 or 4 ohms-the middle and right) each to the 16Ohm input of the cabs and the cabs should be seitched to mono.
The JCM1960 cabs are either 4 or 16 Ohm when switched to mono. Depending on how you are running the head will determine which jack on the back of the cab to use as each jack is labled. Switching to stereo splites each side to an

hm resistance. Your stereo choices are:
1. two separate heads with an

hm output
2. a stereo-output amp or
power amp that runs

hms per side
3. switching a mono amp with two speaker outs to the 4Ohm mode and running a speaker cable to each input jack of the amp. (this is just a waste of a speaker cable and isn't stereo if the amp isn't stereo. this would give you the same result as switching the cab to mono and using one cable into the 4Ohm jack.)
Mike 777 Haug