This has been discussed on previous threads here and herehttp://www.reddit.com/r/AskHistorians/comments/1dujyk/has_the_birth_of_twins_ever_complicated_lines_of/.
Though as ever, people may have information that wasn't in those threads.
In 2008, the heir apparent to the Danish throne had twins as well. Their succession is based on the one born first
This is in the FAQs, as was mentioned when it was last asked ooooh five days ago.
http://www.reddit.com/r/AskHistorians/wiki/royalty#wiki_how_did_inheritance_work_with_twins.3F