chihauhaus aggressive behavior

Question
Have a 1 year old and 9 mo old chi they get alone pretty well except when the 9 mo old wants to sleep.  She become a devil dog and growls and shows her teeth to the older chi The 9 mo is shy and gets very nervious during this period of showing aggresion.  I have tried everything and nothing is working. We love these dogs and want then to be happy with one another. there is no other aggression show at any time except sleep time. Can you give me some advise and please don't tell me there is nothing to be done..both have be spayed  Thank you

Answer
The obvious answer is, keep them separate when the younger one wants to sleep.  If you want the slow process, you could try treating the younger one while it's calm and the older one is far enough away (that it's not bothering the younger one).  Slowly move the older one in closer and reward the younger one while it's still calm and not showing any aggression.  If you get too close and the young one starts to growl, back off the older one (until the young one is calm again) and then treat the young one for being calm.  The hope is that you can slowly desensitize the younger one to the older one.