Yes. The Vernier Arduino® Interface Shield (BT-ARD), has two digital sensor ports. This allows for a Digital Control Unit (DCU-BTD), to be connected to each port, for a total of two DCUs.
When using the shield connected to a SparkFun® RedBoard with cable (ARD-RED), and then connecting the DCU to the first digital port, the DCU is controlled by the Redboard’s digital output lines D2, D3, D4, and D5.
When using the second digital port, the DCU is controlled with the Redboard’s digital output lines D6, D7, D8, and D9.
A single DCU has 6 digital outputs, but only 3 can be controlled simultaneously. Therefore, using two DCUs would provide you with a total of 6 lines that can have totally independent control.