Okay, I went right into MT to test the bars thing.
I took the two square files to crop them identically and made a 2 image bar.
These are the exact images I used:
First thing, this is a capture of the dialog, which shows several things that are not right:
As you see, the images appear stretched down to fill the whole preview square.
I have to say that on the top position, this 2 image bar I made works perfectly. So good enough for me, really, I could move from a multiple images bar to a 2 image one.
Then I put the bar on the right, as shown, and noticed the preview on the lower part didn't change. I tried the slider and as shown, it still treats the image as a block, emptying it vertically. This can be seen in the cap, as I tried to adjust the bar into a point in which the visible part is slightly filled. It's easy to recreate though.
I'll try to test this more thoroughly when I have time though.