My b/g twins are 20 mos. There is some lag in their development but they understand plenty that is said to them - for ex., you can say 'It's dinnertime! Do you want to get in your high chairs?' and they'll immediately scramble off to their chairs. Or recently, I said to one of them 'XX toy is behind you! Turn around and get it.... on the floor'. And he immediately twisted round and picked it up.
So they have pretty good understanding.
Their speech is less developed but I guess that'll catch up.
But here's the thing - they both call each other by one twin's name. E.g. if you ask them where "Millie"* is they both point to the other. Girl twin calls herself "Millie" but will also point to her brother and call him "Millie". He accepts this.
We have always called them deliberately by their own names. Is this a stage they grow out of or are mine particularly confused???
*Not her real name!