Why are there 2 sets on this cover

This is known as a gutter block - in between each pair of stamps is an unprinted blank "stamp".  Great Britain has long printed such a variety in their stamps - a particular collectible is the "traffic light gutter pair" in which that blank stamp in between has coloured dots on it, representing the colours used in printing the sheet.

You can find more information on stamp gutters on this page on the Golowe's Collector Stamps site.

Ryan

I suspect there is no easy answer.

With a single strip of the train , the printed illustration would have been completely visible , the cover conventional and probably more balanced appearance.

The issue dates from the period when gutter pairs, gutter blocks and traffic lights in the gutters were popular things to add to aGB collection. I guess the person creating this cover just had a fancy to put the gutter on it. No reason other than it was possible and took his fancy.

It makes the cover different , the date stamps are on the gutter nice and clear, pity it hides the illustration .

All collectors are free to express their interests as they wish , as long as there is enough postage , Royal Mail will service the cover and cancel the stamps, ( well they USUALLY cancel the stamps, but that is another story. )



If I recall correctly at that time it was possible to get the cover without stamps a week or so in advance of the issue, buy the stamps at any post office on the day of issue and get a local handstamp day of issue. 
In this case Lowestoft . 

now with Post office closures most FDCs come from the Philatelic Bureau or are prepared by one of the specialist GB cover dealers  , Benham Etc