If you have, both, a sticker and a switchable hardcase transponder in your vehicle, only one will be read.
Some tolled bridges, lanes and roads in California offer discounted rates or free rides to carpooling and/or high occupancy vehicles. When driving those facilities with two transponders, the appropriate toll will be collected based on the settings on the switchable transponder if a discount is applicable. More information on carpooling with FasTrak.
On The Toll Roads, it doesn’t matter which transponder is read for toll collection because discounted tolls are not offered and every vehicle must pay a toll.