For International flights, each passenger may check up to two (2) pieces of luggage free of charge. Both pieces must not exceed 20KG (44lbs) in combined weight or 158cm (62") in dimensions (height + width + depth). Please see luggage allowances for sporting equipment below.
EXCESS LUGGAGE: Excess luggage (this is inclusive of weight and size restrictions) will be charged $40.00 CAD or $40.00 USD (plus applicable taxes) each for the third (3rd), fourth (4th) and fifth (5th) pieces checked, the maximum that any one person may check. Carriage of the excess piece(s) is subject to space and weight availability.
So, If you and DH each had 1 checked bag,of 44 kg. you could add a sports bag, free of charge.
But, if you have 2 suitcases each, even if the total of your 2 is 44 kg. the sports bag, is your 3rd checked piece, so you get to pay for it.
This agrees with what you mentioned above; Your replacing one "Item of luggage", but not the weight , that can sitll go in your other bag.
One thing I will mention, is that this year most airlines are being very careful that sports bags only contain specific items.