Opto isolators are not linear (they don't work for analog).
i want to add an ec meter but i can already imagine the ground loop problems im going to encounter.
Isolated or not you probably can't run them both at the same time, so you'll have to power them on one-at-a-time.
i cant find any isolator that handles more than 200ma.
Right. Most opto-isolators are for switching digital signals, not power. What's 200mA for?