$wpdb->prefix DB selection

Quick question, not had any problems with MultiDB and plugins or themes so far until now. Been debugging the SupportPress theme with WooThemes to try and find a solution, but the theme was/is having issues pulling data from two databases with the following function:

$watchers = $wpdb->get_col("SELECT DISTINCT user_email
FROM ".$wpdb->prefix."supportpress_watching_tickets as watch
LEFT JOIN $wpdb->users ON watch.user_id = $wpdb->users.ID
WHERE watch.item_id = $item_id;");

When this runs it pulls the data from the site’s DB, but doesn’t look in the global users table for the users. Instead it tries to pull data from the current site’s DB with a wp_siteid_ prefixed user table. We’re at a loss, so I just hardcoded the proper query for now. Was just wondering if there was some special way of calling from two separate DB’s when using MultiDB or if it should just do it automatically? I assumed it was all done by MultiDB but this one is strange. Not willing to go to a Dev to fix as WooThemes themselves can’t fix it and hardcoding it has fixed it for now, be nice to get some closure on it if possible though.