My (mostly tongue-in-cheek) suggestion was to add a capacitive sensor, not a capacitor. That would add a third sensor (probably) capable of sensing a plastic bag sliding by it. While you're at it, you could also try a laser proximity sensor. 
All types of sensors may be confounded by two or more bags hitting nearly at the same time or sliding down the ramp together. Unless you can somehow guarantee that the bags will be separated by a "large enough" gap, your software will have to be capable of distinguishing between sensor data for one bag and multiple bags.