may be a stupid question, but will they interfere with each other? our house is 3 storey & we cannot hear the kids shout whatsoever when we are downstairs. DS won't come out of his room at night so we still use his & DD will be going in her own room in a few months so we will need a second. We have a BT digital monitor 300 & would prob get another if it will work.
I'm not sure but if you call the number on the box I'm sure they'll be able to help. I rang them the other day as we had a problem with the same model and they were very helpful. I seem to remember the menus weren't very intuitive but I got there in the end... Left a message and someone who knew the monitors called me back.
You will need to check if they can be set to use different frequencies, if yes, then great, if no then they will interfere with each other. you might be ok with it if only one sends a signal at a time - eg only one baby cries at a time, but it might cause cracks and pops and fuzziness- like when you're in your car listening to a local radio station and you cross over into an area where a different local station transmits on that frequency.