Not quite what the OP had in mind, but some phones are coming out that allow two numbers to be used on it! Not sure where/when it will be released, but Acer has this one:
DX900: This phone has dual SIM slots, allowing for two separate phone numbers. It also has 3G a 3 megapixel camera, accelerometer, and a 480 x 640 VGA display.
Found here:
http://www.phonescoop.com/news/item.php?n=3977