Sharing the output from one oxygen sensor to two or more monitoring systems on a rebreather is a popular way of increasing redundancy (and complexity).
This does not cover cell failure issues or corrosion problems but if done correctly allows monitoring redundancy to be added in a shared cell monitoring system.
A few commercial options are available from the likes of Revo, Narked at 90 and Ocean Opportunity. Some include a single resistor, some use two resistors I thought an experiment was in order to check what the difference might be between the single and double resistor approach.
I used an old home made PP02 monitor I made some years ago courtesy of:
http://www.ppo2.com/Displays_1.htm
It uses a very sensitive DPM and has three digits after the decimal place unlike most modern dive computers (the dancing digit).
Three circuits were tested, the first with no isolation to prove the need for it, the second with a single resistor on each split and the third with two resistors on each split (one on the +ve, one on the -ve) the results can be seen in the video below:
All tests were done in air close to sea level, results may differ with increased PP02
The display was re-calibrated each time a set of resistors was added.
The best redundancy comes in the form of 3 cells and 3 separate monitor systems like the original Kiss Classic displays.