Is it possible to specify which edge is used when content flows across multiple pages.

For example, spanning over the bottom edge happens automatically:

+--------+
|        |
|        |
|        | Page 1
|  ***   |
|  ***   |
+--------+
+--------+
|  ***   |
|  ***   |
|        | Page 2
|        |
|        |
+--------+

I want an element to span two pages over their long edges, for example:

+--------+ +--------+       
|        | |        |       
|      **| |**      |       
|      **| |**      |
|      **| |**      |       
|        | |        |       
+--------+ +--------+       
  Page 1     Page 2

Is this possible?

This is not possible yet unfortunately, although we would like to add this feature one day.

Oh, well.

In our case the element is an image, so it might be possible to use two <img/> tags, one on each page and position and crop the elements so that they appear side-by-side.

Yes that would work, or you could try using the @right-middle and @left-middle page margin boxes.