Are you old enough to remember the Alka Selzer commercial jingle, "Plop, plop, fizz, fizz, oh what a relief it is"? I'll bet you didn't know that that simple tune doubled the sales of the fizzy antacid, almost overnight. According to Seth Godin, prior to that commercial, most people only used one tablet.
One of my favorite stories is the one of the factory worker at Procter and Gamble who suggested that one word be added to the instructions on the company's shampoo bottles. The word was "repeat", as in Lather, Rinse, Repeat. Again, that small change doubled sales.
What could we do in our businesses to accomplish something similar? In my days as a stereo store manager, we used to make really nice profit margins on phonograph needles (For younger readers, a phonograph was a prehistoric version of the CD player). I trained my sales people to suggest that customers buy two, one to use now and one to have around, just in case. I also paid a minimal commission on one needle and a substantial one on two. Our sales doubled.
Consumers will often buy two of something. All we have to do is ask.