What you need is a
device similar to this unit. The problem you face is really two fold. As I mentioned earlier and Omega confirmed today on the phone is that the SMS device has analog inputs that share the same common, called a single ended input. If the SMS device had differential inputs there would not be a problem but it doesn't. Next, the Omega pH and mS transmitter is a bit unusual because the + are tied to the power positive and not a common negative. All of this could really have been better planned out but that is after the fact since you have the parts already. What you really need is a dual isolation system where the inputs are isolated totally from each other. The item I linked to is an example. I would think Omega makes such units and am surprised they did not suggest one. Yes, isolators of this type are expensive, unfortunately I don't see an easy way around it based on what you already have. Again, the problem is really two fold.
Ron