DS 5wo and very small, so not very loud crying, and I would like to see if he will fall asleep using white noise so need to be able to hear him over that, he is upstairs in the room furthest from kitchen - safest to get another monitor that is close to him yes? Sorry if this seems a stupid question but if he is upstairs then I'd rather hear too much rather than too little. This is mostly for use in evening when I'm cooking etc. 2 monitors seems a bit bonkers but DD still waking a lot so need to hear both.
How old is DD? Usually by the time number 2 comes along, number 1 is well able to make themselves heard! But if she's still very small or you have a big house/thick walls - then get another one. I can highly recommend the video display ones - hours of entertainment very good for safety.