Whilst A is juggling 3 clubs, B will go on their right and take an arriving club by placing their right hand above A's. The important thing then is that A and B remain in a constant rhythm. B has to throw the club again under A's right arm so that it comes back then to A's left hand. Actually, B replaces A's right hand for one catch and throw.
By saying "half sharing", I mean that each juggler manages one club out of two. It is a continuous "steal and give back" of all the clubs from one side. Considering the above situation, B takes with their right hand all the clubs on the right side of A. A will take with their left hand all the clubs coming from the left.The non-juggling arm can be put on the shoulder of the other juggler and can be used to take and give back a hat/cigar/drink travelling between A and B.
Here B takes 1 club out of 3 (always the same club) and A juggles 2 clubs with a hole (in a 3 club pattern). So B starts stealing and giving back a club as described above. Nevertheless he has to recover this club on the other side with his other hand by going quickly behind A. He then starts the same movement again. So B ends up going back and forth behind A's back, throwing and catching the same club.