In an epub, how to merge 2 html into a single html

Calibre 3.22.1

Hello,
I'm a newbie in epub editing.

I know how to split an html file of an epub at a precise point and generate 2 html files, using the fraction button.

How to do the converse, aka how one can merge 2 html into a single html (regrouping html into chapters and sub-chapters)?

For example how to regroup 0005.html and 0006.html into a merge file called 0005plus6.html ?

Thanks