For flourscents lights with a separate ballast, almost all tube types, you need special ballast to be able to dim them.
I suggest that you have the fixture rewired so that the every other fixture or ever 3rd one is on a different switch.
You might also be able to require so that only part of the bulbs in the fixture are on one switch.
That might require replacing the ballast. But it all depends on the type of ballast and the type of fixture that you have now.
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