I have a sub-album that could be displayed in either of two different albums. I've created the sub-album in one of them, but can I now also display it in the alternative album without creating a clone?

That is not possible because Zenphoto's gallery/albums are filesystem based, e.g. physical. A album is a folder on the filesystem. Either you create two same albums which is nonsense and bad for many reasons.

Or you use two dynamic albums whose search queries match so they both include that album.