It depends on one's travel budget. It is worthwhile only if one + companion are ready to pay the premium to fly business class and provided further that the cost of 2 x discount business class > 1 flexible business + ½ flexible business class by at least 15%.

Years ago, both Amex AeroplanPlus Platinum and CIBC Aerogold offered buy 1 get 1 free at the “C” fare, which one class below the most expensive fully flexible J fare. It was a wonderful deal and was essentially flying business class at half price.

I assume the TD buy one get the second one at half price is available to anywhere Air Canada flies except code share.

With Air Canada, it's probably moot as on some routes, its cheap Flex fare is more expensive than other airline's first class! This is what I have found out in a forthcoming trip to Florida to warm up. I save >$100 per person by flying first class on Delta instead of economy on Air Canada Rouge. Go figure and make sure to shop around.