Which files and folders from the two solutions must be merged to combine web sites solutions

[Cal Who]

If I understand what I'm reading on the Internet the only way to combine two
website solutions and store them in one folder on a host is to make them
into one website solution. And there are certain files and folders in a
solution that have predetermined names and there can only be one of them.

And that means combining some files from two folders into one folder (like
the App_Code files) and combining some data from two files into one file
(like web.config)

It would be great if I knew which files there can only be one of, as well as
which folders there can only be one of.

I believe I know how to handle multiple web.sitemap files.

I believe I need to come up with one web.config file that both will share.

The sites will be essentially independent so I think I simply have to make
sure the master files have different names. Do all master files have to be
in the top most folder?

So to restate the question: Which files and folders from the two solutions
must be merged in the combined solution?

Thanks