Multi high input impedance chemical electrode buffer circuit
I understand you to be intending to add a new pH electrode in its own glass tube, to be monitored by "U3", and you wish to share the reference voltage produced by U1.

This is potentially acceptable BUT pH measurements are so very very very very susceptible to outside disturbance that you need to weigh up the cost gains compared with the potential for cross coupling between the op amps and probes in some way.

As long as the actions associated with one electrode do not affect the other in any way at all it will probably be OK. Can you guarantee this? 



This LMP7721 application note is liable to be of use It relates to an evaluation board but covers much practical material. 



In a pH sensor the output voltage is the voltage developed BY the glass due to hydrogen Ion concentration differences. Sensor impedances are in the order of semi-infinite - say 50 to 500 megohms range, so ie 100's of megohms load impedance is really lower than desirable. 

The op amp cost $US10/1 at Digikey. See data sheet for why it costs so much :-). 

Wow!!! What a superb op-amp - in some areas at least LMP7721 datasheet
 3 FemtoAmp bias current :-) !!!



Related:

pH electrode performance

Wikipedia Glass electrode good

Combination electrode

Single and double junction electrodes

ISFET solid state pH sensors -  - The future of pH sensors, once they get them properly sorted.