Sorry, for being not precise enough.
I'm talking about the the overlay-plugin: https://www.dokuwiki.org/plugin:overlay. By this it is possible to enable an overlay-window for i.e. an index-page. By pressing a button like the edit-Button the hovering window opens and displays the content of a defined page. The name of the page is "overlay.txt".
In the configuration one can define a default-overlay-page, which is shown, if no overlay.txt is found in the viewed namespace. If an overlay.txt is found in the current namespace-directory, then this is displayed instead.
My question was aiming at a second overlay-window, which is handeld the same way but with different content. By this I could use two separate and independ overlay-windows with content corresponding to the viewed dokuwiki-page. I could open an close the windows as desired.
Which changes are necessary to develop a overlay2nd-plugin?