I'm not quite sure what you are trying to accomplish other than something which doesn't actually exist.
If you want such a "Split screen" for playing the game normally, and having one viewport on sonic and one the normal camera, you will be disappointed to know that the movie on the website was created via the following:
- Make an AVI using the non-camhack emulator
- Make an AVI using the camhack emulator
- Merge the two AVI's together using whatever program you want (I used an AVIsynth script). Sound generated by the camhack emulator is a bit weird so discard that audio track and use the original one instead.
- Optionally compress it for uploading.
Also as I previously said, with cam hack enabled the emulation is pretty slow and the sound sucks, so even if you decided to load up two emulators and have them both accept input at the same time you would have to get them running at the same speed (which would be slower than realtime).
I should probably summarise this all: Split screen doesn't exist. It was made by sticking 2 AVI's together. Camhack doesn't play full speed. Making a splitscreen emulator wouldn't be very feasible.