I have noticed one release for which two digital versions have been added, simply because the album has been co-released by two labels. I think this is kinda weird, so I wanted to check here what is the convention in such cases.

Suppose that an album has been co-released by label A and label B, at the same date. Obviously, the album will be for sale digitally on both bandcamp pages of A and B. But this is effectively the same release of the same album, so even if the buy/download links are different, I think it should be considered as one single digital release. Therefore, just like for the CD version for instance, I would expect that there is one digital version on M-A, for instance under label A, with a note specifying this is a co-release with label B.

Is my interpretation correct?

For the record, here is the instance I found where two digital version have been added, one for both co-releasing labels :

https://www.metal-archives.com/albums/Ah_Ciliz_-_Chiral/Origins/692574

You are correct. Report these duplicates (digital or otherwise) for deletion whenever you find them, please. I've dealt with the one you linked and sent the user a message,

The standard is indeed to have one label in the label field and mention the rest in the notes. Unfortunately it's not currently possible to have multiple labels attached to a single release, so that's the way we deal with it.

Alright, thank you!