I don't think you need a different line, just a different feed into the same lines. You can easily have two machines feeding from four lines, two for people with carry ons that have to be checked, and the other two for people with no carryons or nothing to be xrayed. Every other person has bags, every other one doesn't. When the line is held up for a search of a bag, take more people without. If they set off the machine or do have something, send them to the end of the long one. Everybody shouldn't be held up if they don't have to be.