Any relatively heavy duty tripod will work.
I have a spreader bar which was an Amvona product (however I couldn't find one on their web store). The bar will allow the mounting of two
(actually, you can mount four) cameras and/or additional tripod heads. The bar itself can be mounted to a heavy duty tripod head. I have an old Davis and Sanford tripod which could support a VW Bug but, weighs as much as the Bug which I will use with this spreader bar. It includes an older but very heavy duty fluid head.
I will often use it with two camera bodies with long lenses by bore sighting the two lenses so they are pointed in approximately the same spot off in the distance. I will then tape cable shutter releases for each camera to the fluid head handle. A 70-200mm zoom combined with a 400mm prime makes a nice duo when used in this fashion.
A much simpler, cheaper and lighter in weight setup would be to use a bar of 1/4 inch by 2-inch aluminum stock, the length of which would be determined by the width of your two cameras.
Manfrotto sells a ready made Gitzo platform for about $73.
http://www.bogenimaging.us/Jahia/product_main_page/site/bius/lang/en_US/cache/bypass?actualPathCategoryKey=1CAT:AAA1:2CAT:BBB1:3CAT:C141:4CAT
222&curMarketId=NONE&curBrandId=NONE&kindOfProductCollectionRequest=productList&isCleanList=true
Drill a hole through the center to enable you to attach it to a tripod screw (you can use any type of nut for this fastening - however a cap nut is probably the best choice) and drill 1/4 inch holes at appropriate places for the tripod mounting sockets in your camera.
Use a 1/4 x 20 screw along with a wing nut to secure each camera. Screw the screw through the wing nut, then through the hole in the aluminum stock and finally into the camera tripod socket. Don't screw it into the socket with any great force because that may hurt the camera. Then tighten the assembly by tightening the wing nut against the aluminum stock.
You may want to glue some thin cork or rubber under where the cameras are attached and where the bar is attached to the tripod head. This will prevent any scratches and will probably give a more secure mounting.
This entire setup probably weighs less than a pound and costs only a couple of bucks in materials.
Bore sight it off in the distance and adjust the elevation and rotation with whatever tripod head you are using. Make sure the tripod and head can support the weight of the setup. I usually don't like to exceed 50% of the manufacturers recommended maximum weight.
The drawback to these setups is that you can only use the cameras in the horizontal position. However, I suspect that you could jury rig one to support the cameras in the vertical position if needed.
