I don't see the harm in having 2 of those piston/diaphragm. But to be honest, you can simply re-use the one you have. I have a BRS as well, I snapped a few photos of the piston/diaphragm. they're not good pics because it's a tight fit to snap pics but you'll get the idea.
I got the unit used so I can't attest to the tube colors being the same as yours, but the red tube leaves the right side of the 3 prefilters and enters the right side of the piston/diaphragm then it exits the left side and runs to the membrane housing on the left. The blue line runs from the right side of the membrane through the diaphragm then over to the DI containers.
In this case, I would remove the blue lines and reconnect them with one of those
John Guest fittings and cut the drain line at a convenient location so that you can connect it through the diaphragm.
After that I would test the unit to be sure it works properly. Lower the waste water float valve and allow the system to run for a few minutes then close off the float valve and ensure that water stops flowing out of the 0 TDS tube. If all goes right, it will shut off tap water at the piston/diaphragm.
Keep in mind that if you're also using a float valve on the 0 TDS line that you no longer have the capability to shut off the inlet water like it was previously. if you want to do that then you will have to purchase another piston/diaphragm and run it in line of the (in my case) the red tube.
I hope that these instructions are clear enough to follow. and I hope that it goes as planned. Please let me know your results.