Two different keys decrypting same content
Most public key encryption schemes, such as PGP, support this.

When you are encrypting  a message to Bob, in fact you are encrypting the message with a random key using a symmetric cipher, then including the key encrypted to the public key of Bob.

$$E_{\text{PK}}(\mathit{Bob}, \mathit{key}) \Vert E_{\text{Symmetric}}(\mathit{key}, \mathit{message})$$

where $\Vert$ denotes concatenation.

If you encrypt the message to Alice and Bob, you are encrypting the same random key to each of their public keys:

$$E_{\text{PK}}(\mathit{Alice}, \mathit{key}) \Vert E_{\text{PK}}(\mathit{Bob}, \mathit{key}) \Vert E_{\text{Symmetric}}(\mathit{key}, \mathit{message})$$

Each of them can decrypt the message.