You would be stupid to pick up two if you knew you were going to throw one away. Food waste is not a good thing.

But - as odd as this might seem - you could offer the second lettuce to someone else. Either someone else leaving the store, a friend/neighbor or work colleague. Yes I know it's unusual to stand up in the office and say "does anyone want a free lettuce?" but it's not only harmless, its positive!

Fwiw, lettuces will stay fresher longer by wrapping in a paper towel and then foil, rather than plastic.