Hi! This picture can shine some light on the internals of the levers:http://hc625ma.org/wp-content/uploads/2018/07/twin_collective.png
A 10x10 aluminum pipe goes through the lever and connects throttle grip frames assemblies together and with the rest of the lever. Each throttle grip frame consists of two parts, that are connected to each other with a pin on the p1 and a slot in the p2. This is a very tight connection (do not connect p1 and p2 frame parts without a throttle grip - it will be almost impossible to separate them, the throttle grip itself reinforces the assembly), and it is reinforced by a 4x80mm screw with a nut, that is press-fitted into throttle grip frame p1. There's also a cable channel for wires, that goes through the whole lever. If you follow the assembly manual here:
you should not have any troubles while building it (it has been written while I was assembling the lever for my friend).
The levers are quite strong (I am mostly using the single one, but my friends have been using a pair of twin levers for a while without any issues). Note that I recommend printing stuff with slightly higher temperatures (205C) and moderate cooling (that depends on your fans, I use something around 60-65%) with 40% infill. I use regular (inexpensive) PLA.
This video shows the single lever and other hardware in detail:https://youtu.be/6FUu73rrABE
Take a look at the pneumatic mod, it's really good:http://hc625ma.org/collective-pneumatic-mod/
Please feel free to ask questions (you can also leave comments on the site)!