Can I mod rewrite to a different document root?

Given a URL:

http://example.com/some-file.zip

The document root for example.com is /var/www/example.com/

I want apache to:

1. If /var/www/example.com/some-file.zip exists, serve it.
2. If /var/www/example.com/some-file.zip doesn't exist, check /var/www/shared/some-file.zip
3. If /var/www/shared/some-file.zip doesn't exist, send a 404

Is this possible?

I've looked in to the Alias directive, but I think this will require me to have something in the URI indicating where the resource is.

For files outside the DocumentRoot, check out the aptly named documentation section on "Files Outside the DocumentRoot" -- you're looking at creating an Alias, and you may be able to do it without a redirect.

RewriteCond %{DOCUMENT_ROOT}$1 !-f
RewriteRule (.*) /var/www/shared$1 [last]