Thanks all. I just downloaded the instructions for the digital Graco imonitor. It says you can change channels to find the least interference. I'm assuming that I could buy two pairs and set both to the same channel to give us 4 units operating on the same channel.

I can buy a version with two parental units but it only has one baby unit. In this case, the "baby" is mobile and won't remember to carry his monitor with him. I think the two pairs will be best of I can set both to the same channel/frequency.