by sharing if you mean can 2 of you each put 10kg of clothes etc into one suitcase to allow you to go to the max allowance of 20kg(from 1Nov), the answer is yes - families do it all the time.
However, when its friends you also have to have trust that what your friend puts into your suitcase is legal !! And dont forget the question at checkin when you are asked 'did you pack the bag yourself'.
If you mean can you each put 15kg into one suitcase (not having got to your baggage limit of 20kg each) the anwser is no as the max limit of any piece of luggage is 25kg and even this would incur 5kg excess baggage costs.
This is my thoughts and hope this is of some help. If Im wrong I'm sure someone will tell me.
Chris