FEP-9098: Custom emojis

Something else that occurred to me re the microsyntax that should be clarified. Does there need to be a word boundary character on the outside of the colons that delimit the shortcode?

In other words, does this render two emojis or no emojis?

:blobcat::blobcat:

?

If there does need to be at least one word boundary character between them, what are the word boundary characters?