I sell these bars and also supply a pair of bars, a connecting plate, and 4 screws, as you see here:
Start by putting the two bars side by side. Remove the plastic stoppers. You can remove the friction pads if you like or you can leave them on. The connection will feel more secure without the friction pads.
Connect the two bars with one sliding platform. From the pair of bars you have 4 platforms. You only need two for the cameras so you can use one of the other two to connect the bars.
The platform can be enough to hold the two bars together, but for extra support, I recommend that you use the aluminum plate to connect the bars. To do this, align the plate. The two outer openings align with the bar holes (these are standard ¼”x20 tripod sockets). Secure the plate with a pair of screws. You can use the larger screws that are easy to tighten by hand, or use the smaller screws. You can tighten the smaller screws by hand or use the the Allen (hex) key (same key as the one supplied with the bars).
Using the smaller screws will allow you to attach a thicker handle between the screws, as seen here:
What I like about the twin bars is that they attach to the side of my compact photo bag so I can carry everything neatly as seen here (everything that you see on the left picture fits in the camera bag as you see in the right picture - I have a separate blog about this):
Here is a picture of me with the pair of the bars, Samsung NX1000 cameras connected with a wired twin remote and 50-200mm lenses, for long distance action shooting:
If you want, you can use two grips to hold the double bar. You can attach each grip at the center of each bar.