I have two PC and bitcoin-qt on both.
On first PC I have a wallet encrypted with my passphrase AAAAAAA and on my second PC I have a wallet encrypted with other passphrase BBBBBBBBB.
I want have all my btcs in my first PC because I want to sell my other PC.
I know that I can send my btcs from second PC to first PC. But I prefer join the wallets.
Can I merge two wallets in one?
No, you cannot merge two wallets.
You can,though, have a single client software managing multiple wallets.
Here is why you cannot merge wallets. Different from what the term "wallet" denotes, there is no such a thing as a wallet in a PC or in any other device. A "wallet" is basically a combination of private key and public key. The coins are not in the wallet. It would be better to describe the situation saying that there are some coins assigned to your public key in the public bitcoin ledger. The ledger knows nothing of the owners of the coins, only the public key. Only the correct private key can spend the coins. As you can see, the only way to allow another private key to spend your coins is to register them to another public key.
The software you have installed on your PC can manage multiple wallets, though. This link describes how you can have a single bitcoin-qt installation using multiple wallets.