Encrypting with public key does NOT require private key during de-cryption!!!

I observed the following curious situation with PGP v6.5.8:

I have a file which I want to encrypt with both
- the public key of another person say Karl and afterwards with
- my private key.

For that I opened the PGP context menu for that file, clicked "encypt" and the PGPshell popped up.
From the upper pane I dragged at first the public key entry line of Karl into the lower "recipients" pane
and then at second the line with my own key pair entry line to the recipients pane as well.

Finally I clicked <OK> and expected that the file is now encryted twice.

Just for fun I tried now to decrypt the file and double clicked on it.

Much to my surprise I am asked by PGP to enter the passphrase for my own key.

Why this?

I expected that PGP informs my that it cannot decrypt it because it cannot find the private
key of Karl (from which I have only the public key in my Keyring).

If I enter my own passphrase the file is successfully decrypted.

It seems to me that PGP encrypted the file only with my own public key.

How do I perform my original plan for a double encryption?

Just to clarify my view. I don't want only to sign (=calculate the hash value of the contents)
but to really encryt the whole file.

What's wong?

Rod