I know that with Easyjet, paying the sports equipment supplement increases your baggage allowance by 12kgs.
Last time we flew we had two pairs of skis in a double bag which weighed over the 12kgs. However we hadn't reached the 20kg main allowance so weight wise we were ok. The lady did ask me how many skis were in the bag and informed me that I should have paid for two pairs but she would let me off this time. The three previous times nobody questioned it at all.
In the future I'm going to take off the bindings (they're the rail mounted type) and put them in the main bag or hand luggage. That way I can get both pairs of skis in a 'single' ski bag and be under the weight limit.
Now can I squeeze in three pairs
On a side note. We are flying with Monarch to Mayrhofen at the weekend on a chartered flight. When I called them to find out how much it was going to cost to take the skis I was told it was all included in the cost of the flight! However Rocketski, the agent, is charging £21 for ski carriage. Sounds like a nice little earner to me