Hm, but as I understood it, in Greek (Ancient Greek, at least), adjectives that are composed of two components only have two distinct endings (-ος, -ος, -ο) and not three (-ος, -η, -ο).
For example, έγκυος because it's a compound of εν + κυ-, so Η γυναίκα είναι έγκυος (and not έγκυα).
So I would have thought that δι- + γλώσσα = η δίγλωσσος γυναίκα.
(But Wiktionary disagrees with me on that word and agrees with you.)
Really? That's interesting. Maybe because I've never heard of "η δίγλωσσος γυναίκα" in spoken greek nor ever seen in it text format it didn't come to mind.
I did search it up after I saw your comment though, and δίγλωσσος,-η,-ο seems to be the most popular one. O.o Maybe δι- is an exception? Like, διπολικός-ή-ό or δίμετρος-η-ο, δίωρος-η-ο, or even διήμερος-η-ο?