Sidebar setup

Hello,
i recently miss a box on the bottom of the page setup, where i could select how many sidebars shown
and where placed.

I have several sidebars defined. My custom hueman theme is set to 3 columns (global layout) -> 2 sidbars, content in the middle.

All other defaults are set to inherit.

For one page i like to see sidebar A, for antoher sidebar B, again another sidebar A and B.

How to select multiple sidbars per page ?

Thank you