I ordered my Kindle Fire and am going to give my $139 Kindle 3.3 to my daughter...
I have around 100 books in it, but, she also has her own Amazon.com account...
Q: can she use two separate Amazon.com accounts on the same Kindle???
No. As sandpiper said, a Kindle can only be registered to one account at a time.
Your choices are:
1. Leave it registered to your account so you can share books.
2. Remove all your content and give it to her in factory condition for her to register to her account.
It is against Amazon's Terms of Service to transfer a Kindle to another owner with your content intact. It's not even possible to do with the Fire and I've heard at least one report that it's not possible on the Basic Kindle either.
If you and she tend to read a lot of the same books, sharing an account can work well. My brother's kindle is registered to my account and it works great. . .he periodically sends me a GC to apply and I keep track of what he buys and how much 'credit' he has left.