@McGuire2016, you won't be able to use the 2 parent units. You'll only be able to use one parent unit with up to 4 baby cameras.
If you go this route, remember that you can buy the cheaper model (2000 or 3000 if you're not bothered with the rotating motor). You already have the large screen from your 6000.
Having said all this, have you seen the post above with potentially a place where to buy spare cameras? Or have you tried 2nd-hand sites like eBay or equivalent?
I'm also trying to add a second camera to my existing 6000 monitor.
I emailed BT and had a very useful reply, saying I could connect a camera from the 1000,2000,3000,4000,5000 or 6000 models. There will be limited functionality in some combinations (as expected, you can't pan-tilt/talkback using a camera that doesn't have a motor or speaker!).
They also said that using a model 2000 camera with the 6000 handset will produce a lesser quality image. The 5000&6000 have better sensors than the 2000 but they seemed to suggest the image will be magnified. Can anyone comment how different/bad this image is?