What is the difference between single and twin lift picks? 

Twin Lifters work better in picking and reading, they read in the same direction as picking direction. However, they can not work on over-picked wafers; once you over-pick a particular wafer then you need to reduce the tension and start over again.

Single Lifters pick in opposite direction to correct over-picked wafer, also they can work in jammed locks. However, they read the opposite direction of picking direction. 

The vehicle application for both tools is the same, so it just comes down to a personal preference. Some hardcore Locksmiths prefer single lifters over twin. Some have both and use twin lifter for picking and decoding an ignition, and single lifter for other operations.