Calling shallows medium is a bit confusing. /QUOTE]
I accept it is a bit confusing and in the UK there are indeed Jumbo, deep and shallow Langtroth frames with the latter being 5.375" deep (137mm approx). However, all the poly hives I know do not use Langstroth shallow but use what in the US are known as Medium or 3/4 Langstroth frames, which are 6.25" (159mm) deep.
Unfortunately, if you ask most UK suppliers for Medium frames they will probably look blank because in the UK to get frames of this depth you have to ask for Dadant Shallow. In theory, Dadant frames will be wider spaced than Langstroth but I have found most are the same spacing as Langstroth, possibly because the manufacturers' machines are set up for this size.
As Onge says, a Dadant shallow frame is almost exactly 50% of the area of a Jumbo Langstroth (or Dadant Brood) frame, so running the colony on two Medium boxes gives the same volume as a Dadant without the weight issue. There are of course more frames to inspect but they are each half the size of a Dadant the so the extra effort is not double.
I only have two seasons experience of running colonies on double Mediums but it is common in some other parts of Europe and also the US which gives me the confidence that it is a viable method of running a colony.