Most of us have apparently decided we can't live without our favorite mobile device. Whether on public transportation, shopping or just walking down the street, you're more likely than not to be surrounded by people swiping screens, adjusting their earbuds or typing on a virtual screen.

But while a mobile device is increasingly seen as a must-have, what happens when one increases to two, three or more? After all, the number of mobile devices owned by the average U.S. subscriber today is 1.57, according to Wireless Intelligence, the research arm of the GSM Association, and 1.85 in the rest of the world. By the end of 2013, there will be more Internet-connected mobile devices than people, according to  Cisco's Visual Networking Index.

According to Google, 90% of us juggle four screens in a day (smartphone, PC, tablet, TV), often starting a task on one device and completing it on another (called "sequential screening") or using multiple devices at the same time ("simultaneous screening").  Nine out of 10 people are sequential screeners, according to Google, while 77% watch TV with another device in hand.