The MaxBotix sensors found the in the kit of parts have a pinout that is detailed here:
http://maxbotix.com/products/MB1010.htm
As can be found on the data sheet, there's not only reading outputs, but pin 4, the RX pin, is internally pulled high. By pulling this low (connecting to ground), the sensor will automatically stop ranging.
If you can control when the pin is pulled low, you can control which sensors are ranging at any moment, and thus alternate readings between sensors to prevent any interference.
Edit: In order to do this, You can create a digital output pin on the sidecar, hook it up to the pin on that sensor, and set it to low (pulling the pin low as a result).