I think you have just answered yourself. Homepage.html in root folder will do the trick. Rename it to index.html so it loads when accessing testdomain.com.
You would definitely have your shop installation somewhere else, not in the root dir.
Give the new page, index.html, 2 links, one to shopA and another to shopB. Of they are on the same domain, consider subdomain?
If it does not load, check webserver config