If they make a Legendary enchantment creature with Bestow, would you be able to cast one as a creature and have one as an enchantment at the same time?
I wouldn't think so. It would be the same as the Gods, even though those don't have Bestow.
You wouldn't be able to control both. The legend rule doesn't care whether its a creature or enchantment:
704.5k (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=704.5k): If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners' graveyards. This is called the "legend rule."
Awww...