- If I have an enchantment creature with an aura attached to it, and I bestow the enchantment creature on another creature, does the aura go with it or go to the graveyard?
- Can an enchantment creature be enchanted by another enchantment creature?
Bestow can only happen from your hand. While the card is in your hand, it is simultaneously an enchantment card and a creature card. When you cast it as a creature, it becomes an enchantment creature. Once the enchantment creature is on the battlefield, it is unable to become an "Enchant Creature" spell (I'm reasonably confident that all creatures with Bestow can only enchant creatures). Because of that, the Aura attached to the enchantment creature doesn't go anywhere because the enchantment creature can't enchant another permanent.
Yes. Again, while the card is in your hand, it is simultaneously a creature card and an enchantment card. When you use the Bestow ability, in this instance, the enchantment creature being Bestowed becomes an Aura and ceases being a creature; that Aura spell becomes a creature and ceases being an Aura if the creature it is enchanting leaves the battlefield.
Bestow is a choice that you make while casting a spell. If you choose to cast it with the Bestow cost, it is an Aura spell instead of a Creature spell. This is a choice that you can only make while casting the spell. If the card is already on the battlefield, you can't turn it into an Aura with the Bestow ability.
You can enchant an Enchantment Creature with an Aura that has Enchant Creature, including an Aura that is a bestowed Creature Enchantment card. An Enchantment Creature is just both an Enchantment and a Creature, so it can be affected by things that affect Enchantments, and by things that affect Creatures, including Auras with Enchant Creature. Adding the Enchantment type to a Creature doesn't otherwise do anything special.