The first TDS meter is plumbed into the first chamber which measures the water entering the system from the tap. And the second meter measures the last chamber exiting the unit. It's a very simple system to figure out. I use the booster pump to increase the pressure to assist with function because the system has to perform at a certain psi.
They were installed in March. And only used to do two 20 g water changes in April. I took down my tank at the end of April....and these haven't been used since then.