[bojan]

Composite video can be combined by simple addition/dividing by 2, using fast operational amplifier, very easy to obtain, but for BW cameras only.
However, the cameras have to be in sync, both vertically and horizontally.
In case of colour cameras, the combining must be done on video part of the signal, so not to touch the colour burst and colour carrier. Attempt to combine BW and colour signal as described above may have quite unpredictable outcome, namely there will be some colour shift as a result of this operation, or the colour information may even be lost.
I am not sure how you can achieve all this with two simple CCD cameras, without sync and proper mixer, which disassembles the composite signal into components and then performs the addition to the relevant parts of the signal and then puts the whole thing back together again.. Actually I think this is not possible without extensive video stream processing, using powerful computer.

However professional cameras, much more expensive should be able to be synchronized. I am not sure of suitability of such cameras for Astronomy, though.