I have the M's and J's in a straight cab, and initially experimented with their positioning.
At first, I mounted them with the J's on top and M's on the bottom.
I was curious, so I flipped the whole cab 180 degrees (no casters), which put the J's on the bottom. I felt that gave me a fair idea of what they would sound like in a reversed position
Ultimately, I mounted them in an X pattern, which won out over the other experimental configurations, and sounded better in my Bogner 4x12. I found that the X pattern offered a fair mixture of both speakers, as opposed to having whichever pair were on top, beating out the bottom pair. I stand close to my cabs when I play though, so that is only my experience.