Is there a standardized way to achieve this?
That's actually a fairly common request - here's the problem - there's no way to do it without making the ciphertext as long as the two plaintext messages concatinated (after compression).
Let us suppose we had a magic box that could take the two plaintext messages and the two keys, and generate a ciphertext that could be decrypted either way.
Then, we could use that magic box to attempt for text compression - we could take our two messages (and pick two arbitrary keys) and generate the ciphertext. That ciphertext is the 'compressed' version of those two texts.
To uncompress them, we just decrypt the ciphertext with the two keys, giving us the original messages.
Because this acts as a lossless compression, it seems unreasonable for us to expect it to work better than known lossless compression techniques.
For example, if the two strings are both random 256 byte strings (that is, uncompressable), then the ciphertext cannot be shorter than 512 bytes.
And, if the magic box picks the keys (in addition to generating the ciphertext), then the limit is one on the total length of the keys and the ciphertext (because then the keys would need to be included in the 'compressed' version...