2 collapsible text in eachother

hi,
i have a feature request that would be very great.

I'd like to put two collapsible text into eachother like

1. Headline 1
bla bla bla bla
1.1 Headline 2
bla bla

So the 1.1. ist collaped and the user can just see it if he opened the collaped text at 1.
Thanks