need some help with a .htaccess rewrite rule..

[edge]

I'm trying to do the following with a .htaccess rule (apache2), but till now with no luck.

test-a.website.com should show website.com/test-a/index.html
test-b.website.com should show website.com/test-b/index.html

I would also (if possible) to still show "test-a.website.com" as URL, and not redirect it to "website.com/test-a/inde.html" (same thing for test-b.website.com)

I guess it's done with a rewrite rule.. but how?

Thanks for any info..