PRSI rates as follows
4% of income up to 46,600
2% on the balance
If your earning less the pension contribution are under the PRSI limit of 46,600 then you get the benefit of the 4 % reduction up to 46,600
In your example
B earns 48K - Pension Contribution 5K = 43K then the 4% benefit only applies to 3,600 (46,600-43,000) of the contributions plus the full 5K at 2%
A earns 65K - pension contribution 5K = 60K does not get the benefit of the 4% reduction but does get the benefit of 2%
In your case you should consider putting the lot against B's income provided this does not bring him/her into paying tax at 20% only.
Don't forget you can still claim tax relief against last years income.