I have to agree with you about the wiring. The instructions take the builder through the mechanical portions of the bild in an almost Ikea-like fashion, and then there is little direction when I comes to the wiring. For my original Burly build, it was a pleasant 10 hours over two days to get the mechanical portions assembled…and then it took me three additional days to figure out what I wanted to do with the wiring and then 3D print some parts. To be fair, there are multiple solutions, and some variation, but I think it would beneficial to have an “official” solution with some pictures for both serial and dual-endstop wiring.
With that said, yes, X1 and X2 are wired in series to the two arms of the serial wiring harness. If one of the motors turns the wrong way (easier to check before putting the wires inside anything), you can just flip the connector to reverse the motor.
As for where to put the wires, there seems to be three solutions:
There are some 3D printed parts around to support all three of these solutions, but they are not part of the official files, and you will have to seek them out.