I phoned 'Flight Crew Licensing' and asked a few weeks ago.
The first response was a rather stroppy
"Well you wouldn't cut your driving licence in half would you ?"
but then I heard someone whispering to her in the background, followed by her coming back and telling me, rather sheepishly, that apparently it was okay to cut it in half after all !
Just make sure you use a sharp pair of scissors.