Valid medical at time of licence application (doesn't have to be valid at time of skill test - but ideally would be).
OF MORE IMPORTANT NOTE!!! If the multi engine CPL skill test that you did, included your Multi Engine Piston Class Rating THAT BIT is only valid for six months - not twelve.
If you don't already have a multi engine piston class rating in your licence (presumably PPL) then GET THAT CLASS RATING APPLICATION IN BEFORE THE END OF SIX MONTHS!!