Not sure if this is the right place to ask, but I'll give it a shot.
How does one do a 'picture in picture' style video broadcast on a local CCTV feed?
We have two cameras, just need to know how to mix the video to get the desired output.
Well I sure know what's involved to do that on a broadcast level but I'm sure today there is a low cost prosumer box that can do this quite simply.
In the old days before the 1990s, both cameras would have to be locked together with a common sync generator and simple security cameras lack that capability. But most switchers today have video frame synchronizers built in that eliminate this requirement.
Look at "seamless switchers" at www.extron.com for some ideas. Their stuff is a bit expensive but their site will give you some idea of what you need.
Note that Ebay has a lot of pro video gear at really good prices since many people get this stuff from liquidation auctions and don't know what it is or what's it's really worth.