FYI, I'm not sure if FTB fixed their system, but for 2021 and 2022, I didn't have this problem. My RDP and I filed California tax return separately. My Form 540 line 71 (California income tax withheld) did NOT match my W-2's box 17 (state income tax) -- in 2021, it was lower (because I made more than RDP) and in 2022, it was higher (because I made less than RDP). Both years, FTB didn't have an issue with that.
On Federal Form 8958, TurboTax did put information on how we were splitting Federal withholding in line 11. But TurboTax didn't put anything about how we were splitting California withholding. So on my CA tax return, I don't see any entry showing how I arrived at the figure on Form 540 line 71, but FTB didn't raise any issues. FTB does know about my RDP's return because box 3 is checked on page 1 of Form 540, along with my RDP's SSN on the top of page 1, so maybe FTB is smart enough to verify that the sum of Form 540 line 71 from both our returns matches the sum of both of our W-2's box 17.
