Eh don't worry lah, with twins memang timing berbeza-beza for each baby and where they're positioned! My friend carrying twins also didn't feel the second baby move until almost 20 weeks while the first one she felt around 16w, so it's quite normal especially if your placenta is in front (anterior). Since your scan last week was normal, baby is definitely fine, but if you're really anxious you can always call your gynae just for peace of mind, they won't mind at all!