dual pH sensor system
I would definately recommend Approach#2. In this approach you are isolating a digital signal, which is a much easier design to get right than trying to isolate 2 low-level analog signals from sensors. Isolation is easy for a digital signal, where you really just need to reliably transport the presence/absense of a voltage above the digital logic threshold. Isolation for an analog signal is difficult, where all kinds of offsets, temperature-related transfer function changes, noise, etc. all influence the measurement accuracy as seen at the ADC. It will be much much easier to design an accurate sensor reading system using Approach#2.

In fact, since the sensors are 6-7m apart, I would suggest Approach#3, where each sensor has a dedicated ADC, and the isolation is applied to each sensor/ADC combination individually.