Airlines will not normally allow people to "share" luggage allowance.
The Ryanair web site says
"Checked baggage in excess of the checked baggage allowance per passenger will be charged a fee per kilo!
More here
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=excessbaggage
Note that even couples cannot share luggage allowance.
Suppose the luggage allowance was 20Kg per person.
If a couple were flying they could not have two cases, one of 30Kg and one of 10Kg. The person with the 30Kg bag would be charged for the extra 10Kg, even though their partners was 10Kg under.