To connect two cameras you need to open the camera and solder some small wires to all relevant pins. Then we apply a pinhead to the wires, fix it on the outside and connect the pinheads of both cameras via a cable so they act as one camera.
What you need is:
two identical cameras (may be old and cheap, I used two Concord5040, for 10€ each)
The simpler the better. Stereophotography works best if everything is the same except for the viewing angle.
thin copper wire (0.15mm diameter)
a solder iron with tin
two pinheads, number of pins depending on how much functions you want to synchronize
two jacks for the pinheads with cables
Mounting for two cameras
Step 1: Open Camera and identify pins
All cameras I know have the case screwed on.
As the screws are very tiny and small, I advise you to work in some kind of box with a small rim. The top of a shoe box is ideal. If anything falls down accidentally it won't roll away and nothing is lost.
If the casing has different screws it is good to glue them successively to some clear tape in the sequence as you unscrew them. This way you can easily keep the track which screw belongs to which hole. I also used small dots beside the holes to identify the used holes.
You could also take some pictures during disassembling, to remember the steps.