Each airline is free to determine their own rules (and also to determine how much flexibility they'll permit within those rules). However every airline website I've ever looked at, when replying to (many dozens of) baggage enquiries here on AB, states that the pooling of baggage allowances is not permitted.
'Full fare' airlines might offer quite a bit of flexibility when it comes to baggage allowances. (Qantas let me off hundreds of pounds of excess baggage charges, simply by requiring me to re-pack some of my bags at Adelaide check-in desk). But budget airlines will definitely charge you a substantial excess baggage fee.
Chris