Here are some screenshots of what I'm seeing hopefully this helps track down the issue.
This is a screenshot of my Campaign Character options that shows that 1) I have the Gamemastery Guide resource 2) the Dual Class option is enabled.
https://ibb.co/NV2j1h1
A shot of the Dual-Class PCs option selected.
https://ibb.co/QNCQwBy
And then a shot of what it looks like when myself or one of my players tried to turn on the Dual-Class option in the characters Modifications --> Character settings.
https://ibb.co/rFn1KHB
Any help with this is greatly appreciated.