Yes. Switchable FasTrak transponder(s) are required on certain tolled bridges, lanes and roads in California to recieve discounted rates or free rides for carpooling and/or high occupancy vehicles. If you have both, your sticker transponder and switchable transponder can be mounted to your windshield at the same time.
 
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. You will not be charged twice. For more information on carpooling with FasTrak, visit FasTrak throughout California.

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.