Mod_rewrite on request to give a page from another domain?

Question

Apache HTTP Server

On Habré, I saw an example in an article in which, using mod_rewrite, when requesting a page, another one was given,
requesting the a.html page, the contents of b.html were displayed,
but is it possible to do this only with different domains or subdomains?
For example, when requesting example1.com/a.html, we get the content of example2.com/b.html ?
By means of mod_rewrite.