So yes - in answer to the original question. It is possible in the Designer Pro to position the thumbnails horizontally in pairs by adjusting the size of the larger image and the thumbnails (and borders, padding and margins) to make sure both areas of image content occupy the same number of pixels in total.
Can I ask for further clarification on this. I have the same problem Sally had, where the thumbnails tile horizontally when I changed the main image size after downloading 3.8.
I've tried adjusting the image of the thumbnails to equal the main image in total, but it doesn't work. I need to make them a touch smaller in total for them to tile vertically, and then there is a tiny alignment issue on the right hand thumbnail.
So this leads me to ask, which padding, borders (only have 1 default for images) & margains would I need to allow for to make their totals equal? I don't want to move my product description page around too much from what it is, but I'm unsure which ones affect the image issue.
Thanks.