share login/logout sessions across two installs?

You also need to define a matching COOKIEHASH for both sites – a random 32 bit string will do.

By default, COOKIEHASH is an MD5 hash of the site URL, and is used to generate the default names for all authentication-related cookies. Hence why, at the moment, your cross-domain login isn’t working (the names of the cookies aren’t consistent, as COOKIEHASH will be different for each site).

See wp_cookie_constants() for more information.

So to recap, your wp-config.php for both sites should look like:

define( 'COOKIE_DOMAIN', '.stolenmx.com' ); // Dot prefix
define( 'COOKIEPATH',    "https://wordpress.stackexchange.com/" );
define( 'COOKIEHASH',    md5( 'stolenmx.com' ) );

define( 'CUSTOM_USER_TABLE',      'wp_users' );
define( 'CUSTOM_USER_META_TABLE', 'wp_usermeta' );