Multiple sensors on a single EC sensor board?

Hi, I am wanting to build a controller to monitor / control a reverse osmosis / de-ionization filter. My question is regarding measure TDS (or Electrical Conductivity). I would like to use the SP-2 probes here, as they are $12.50 for a pair of them, and are perfect for this application:

http://www.tdsmeter.com/products/sensors.html

I would prefer to not have multiple EC sensor circuits, as I would like to monitor up to 4 different points in the process. Here is a potential EC circuit, although there are others (atlas scientific for example)

http://webpages.charter.net/tdsmeter/

My question is whether or not it would be possible to connect multiple sensors to a single EC circuit, and if so how? Could I just connect them using a relay, or is there a better way, or is it not going to work?

Any help is greatly appreciated.

Thanks