Hello, this thread is quite old which means some of the product recommendations aren't that relevant anymore. These days most baby monitors are digital, so you should be able to use two at the same time without any interference. Another option would be to get a baby monitor which can be connected to multiple baby units, this means one parent unit can be used to monitor multiple children in different rooms - genius.
I still use a video baby monitor with my 14 month old! I like to see what he gets up to when he's meant to be going to sleep! I have used 2 monitors in my house when my friend stayed with her dd. It worked fine but they were on different frequencies and different makes of monitor. It might just be a case of fiddling around with them til they both work without interference. They usually have 3 frequency settings so should be fine.