My little guy did that too. He’s now 13 weeks but he would smile at 2 hours no problem, and then crash. I noticed he does this if he falls asleep while breastfeeding. Are you BF or bottle-feeding? Another thought is that they could just not be showing obvious sleepy cues. My little guy would talk more and try to engage even when sleepy.
Maybe try to do a wind-Down around 1hr-1hr15min mark and see if they start getting sleepy. A dark room or quiet singing / activity. Do you have a nap routine?
Congrats though on your twin babies! Lots of work but keep it up!