I also have two. One a 'regular' sound one in the room with the little one. Otherwise I can hear the others scream quiet clearly! She whimpers sometimes and that I wouldn't hear and if we're in the playroom while she napped again that's 2 floors away.

Then I have the video monitor that used to be in the little one's room so that I can get used to her sleep habits & cries of 'dreaming' vs. upset. Now it's in the basement to keep an extra set of eyes on the others while I do potty/diaper runs and nap the little one etc. They still don't get how I know when they're touching something they shouldn't! LOL