It's not a question of 'frequency', they need to be solidly syncronised as well - and domestic equipment doesn't have such facilities.
As we've told you, it's not easily or cheaply possible - you need to digitise and memorise every single frame, digitally rescale them both, join them together in the format you want, and them output the resulting combined frame - all in real time.
So, you're basically saying that an Amiga with Genlock can't exist today - interesting...
The truth is, with a PC, all of the above is easily possible, and fairly cheap (as long as you're not going out and buying a new PC). Get one of those multi-input DVR cards, there's your digitizing hardware; if you get one that is based on the BT878 chipset (as well as a few others), you can drop a copy of Ubuntu on that box, then load up ZoneMinder and have a -very- professional security camera/DVR/monitoring solution like you wouldn't believe. ZoneMinder and Ubuntu are both free, by the way. I've got it set up right now on my workstation, monitoring a WiFi camera that points out the window of my house. When someone walks by (I have custom zones set up in the camera's view to adjust the sensitivity and area of the image I want to monitor), it analyzes the video, time-stamps the images, records a video of the action w/timestamp overlay (5 seconds behind the trigger and 5 after), emails me a picture and a video to my cellphone, and stores everything in a mysql DB, for later recall, playback, etc. I have only one camera, you could have many, many more (it also supports PTZ mounts, external triggers, etc). Oh - everything is done via web interface, too; your cameras don't even have to be on the same continent as the server (latency and such would suffer, though - so keep it on the same subnet in a different part of the city at least).
Seriously - you could take your favorite Linux and ZoneMinder, put them on a 1U server with a small RAID array, and sell it along with a bunch of WiFi cams as a custom, turn-key security solution.
Of course, I will say that I don't know what
Zephyr's real application is; it seemed like security monitoring.
Ultimately, though, any "PC" made in the last 15 years could do what you're talking about, and if you went back to 1985 - the Amiga could, too (although, in 1985, getting such a setup would be fairly costly, though certainly much, much cheaper than the commercial video processing offerings at the time). One would think someone from the UK would know that...
Today - find an old PC made in the last 5 years (heck, I've probably got 5 sitting in my shop here at home) - drop a cheap video capture card on it (quad-DVR cards are cheap), and run some software (most of those cards come with something for Windows; you can also find other software offering out there - my preference has been with ZoneMinder on Linux - which is a free and -very powerful- security camera solution).