We had a similar situation with J and R. R was never a vocaliser for such a long time whereas J never stopped talking from the moment he learnt he had a voice box. I was worried about R's speech but my GP told me to worry when he is 3 and not saying anything at all (and partial-sound words count!). He would just grunt or mumble unintelligable things occasionally. Turning two was the turning point! Its like he just suddenly flicked his vocab switch and now he doesnt stop talking. So maybe Ds2 is just happy to let Ds1 and everyone else do the talking while he listens and does things. R is very manual; he loves using his hands, building with blocks, climbing, riding his bike. We figure its just his thing.
I hope a speech pathologist gives you some answers or Ds2 just surprises you in his own time! Never wrong to get concerns checked though; better safe than sorry down the track.