Wiring two humbuckers (50s Les Paul®)
When using shielded coaxial wire, the outer braid is connected to ground. The inner wire is the hot conductor to carry the signal.
Alternate volume control wiring: You can wire both of the volume controls of a Les Paul (or any other dual volume control instrument) so that you can blend the volume of the pickups independently. This solves the problem in the middle position of the selector switch where turning down one volume control affects both pickups.
Pickup height adjustment
For humbuckers, there should be about 3/32" gap between the top of the low E polepiece and the bottom of the low E string fretted at the highest (21st or 22nd) fret, and about 1/16" for the high E. Humbuckers can typically be adjusted closer to the strings than single-coils, since they don't have as much focused magnetic pull. Experiment with the height of your bridge and neck pickups to find the best balance in output and the best tone.