At All4Cellular, buy a select cell phone accessory and get a second one free. (Add two items to your cart and the discount will show in-cart.) Even better, coupon code "A4C5OFF" cuts an extra 5% off the pre-discount price. With free shipping, many duos are at or near the lowest total prices we could find. Choose from earbuds, cases, stylii, and more.

A best bet is the Happy Heart & Diamond Case for iPhone 4 and 4S in Silver/Pink (pictured) for $2.95. Add two to your cart and the total price falls to 90 cents before the above coupon, 60 cents after. With free shipping, that's $0.30 per case, $5 off, and a very low price for two such cases.