Another theory is the ship that went down with Elsa and Anna's parents was on its way to Rapunzel's ceremony after she was found. To pay their respects, three years later, Rapunzel and Eugene went to Elsa's coronation. Funny thing is, Frozen was released three years after Tangled. Coincidence? I think not.
Also, I read on tumblr that Elsa and Anna's mother and Rapunzel's father look significantly alike and might be brother and sister. Which would explain the visit, but it's just a hopeful theory.
A lot of Disney movies are connected and not just in a "making a minor reference" way. There should be a post about it somewhere on the internet. Might be this one: http://cdn.arwrath.com/1/130218.jpg
I think you meant *confirm