A recent Steam Controller update (possibly beta only update) added a second Bluetooth pairing slot, letting you pair two Bluetooth devices (just like how you can pair two pucks). To start the controller in the second Bluetooth slot, you turn the controller off, and then hold L1 + B + Steam to turn it back on (versus R1 + B + Steam for 1st BT slot, or R1/L1 + A + Steam for the two Puck slots).