Get a nice dop on your iron, then gently place it with iron between them. The solder while not flowing will help transfer heat. If you start with iron on both you may not transfer enough, and pushing solder in is risky because it might not make contact. You may end up doing both, trying to have a lot of solder in there. It looks like Gary did this, cause it ain't pretty. Try to disconnect anything you don't need connected, that's easy to remove.