You can do an experiment without bothering with Custom Inputs. Just set it up as a regular ADS1115 as you have done.
- Connect both pH and TDS probes to their sensor boards, and connect each sensor board output to two channels on the ADS1115.
- Make a voltage measurement of the pH sensor output with the pH probe only inserted into the solution. Do not insert the TDS probe into the solution. Note this voltage.
- Now insert the TDS probe into the solution, and see if the pH sensor voltage changes.
If the pH sensor voltage changes when the TDS probe is inserted, then you are seeing galvanic effects from the TDS probe. You will need to solve this first.