As far as I am aware under EASA, ONLY aircraft that are certified as needing two crew can accrue 'multi crew' hours. If you are flying an aircraft that is certified as single crew only then I have a nasty feeling that those logged hours won't count towards the multicrew requirement for your JAR ATPL. So to unfreeze your JAR ATPL you will still need 500 hours multicrew time on a multicrew certified aircraft.
I know that under FAA you can log the hours for two pilot operations in single engine aircraft, under JAA you can't.