Why does my double cart break apart here?

I have a double cart booster which I am attempting to set up. As you can see, the cart breaks into two on the return.

A full view of the track, and a close up of the section missing from the video is here:

EDIT: The moment the split happens:

I don't know why the double cart fails when hitting the dirt block there, but I'm not going to give too much thought about it, given that it's a glitch we're talking about.

Replacing the end of the second booster with another circular return would let the boosting carts retain enough momentum to reset all the way to the first booster and the booster cart gain enough momentum for your evil needs. Given that the first reset consistently didn't break the double cart, this should also be safe.