Same Login for multiple WP installs

I have 2 WordPress installs on my domain: http://example.com/ and http://example.com/subfolder/ (No WordPress Network!)

Is it possible that I login on http://example.com/wp-admin/ and if I visit http://example.com/subfolder/wp-admin/ I’m already logged in?

Both have already got the same CUSTOM_USER_TABLE and CUSTOM_USER_META_TABLE and the same secret keys.