One from A-master sheet(initial hide) and another lightbox in the local page with Auto-lightbox?
Drag the widget with the composition lightbox on your master, make your settings as desired, change to your page, drag the desired widget there and you should be pretty much done.
The light box from the master will show up on all pages, related to this master, of course.
In case I am missing something or in case you forgot to tell us what you want, provide a screenshot or even some more details.
Pls see the screenshots you figure out my problem.
I tried giving z-index also
its not working.