Interesting scenario, NorCal.
Using my model of “one lightning target, one ground electrode,” you NEED use only one.
When Mr. Ufer invented his electrode, his goal was to ensure excellent contact between the electrode and Mother Earth. A wire within poured concrete set upon the ground provides as good contact as you’re ever likely to have.
What hasn’t been foreseen is the possibility of a building having a split foundation— which is what you describe. Trade practices would have the foundations solidly connected to each other, but those connections would not involve bonding the rebar together.
Now, for the sake of discussion, let’s consider an industrial building with an ungrounded service (no neutral). Such buildings typically have excessive bonding of the building steel and machinery to the ground electrode. This is one scenario where having such a split foundation might result in some unusual electrical problems.
Another possibility is someone later bonding to the unused Ufer. That would violate the requirement that all grounding electrodes be bonded together, possibly creating a potential difference between parts of the same building.
So, while code doesn’t explicitly require both Ufers be connected, I would consider such to be good practice.
