DD, only child, is 21 months and not doing as much as OP's, even though she's very verbal.
Only two-word English sentences I can understand so far are bye-bye something or night-night something. However, she says lots of "sentences" that I don't understand at all.
She does seem to be learning a word or two each day at this point. We spend a lot of time each day where she points to something, and I tell her the word for it. . . but she doesn't usually try to say it. (This is a game she started about 3 months ago on her own). And in the past month, she's started pointing out connections. . . so she looks at a picture of our dog, says her version of the dog's name, then points in the direction of where the dog is. . . or the car, or the oranges, or the window, etc.
Articulation is terrible, to the point that I'm considering writing out a little guide for other care-givers (especially my mom, who will be watching her when her sister is born) so they can understand her.
Her comprehension is great. She knows the things in most of her books and can point to them on command, but she doesn't try to say the words for more than a third of them. She regularly follows instructions like "find your shoes and put them on" or "put that back where you found it."
Jo
DD#1 "JellyBean" 6/08
DD#2 "Ha" 6/10
"Theory is great, but as I'm in the trenches of diapers and dishes and ear infections, I try to relax and focus on what's most important: love."--mjs64