Dan Pierce said:
The plus and minus wiring is relative to how the speakers are placed. If they face the same direction then the above diagram is correct.
If they are back to back then this wiring will cancel out all the base as the speaker cones will move in opposite directions thus creating no sound.
Maybe, but maybe not…, and maybe the opposite.
He intends to put them in separate enclosures, so the back pressure of the cone from one speaker has no effect on the other. Because the the speakers may face downward, or in different directions, the only phase that then matters is if the sounds of the speakers are in phase when they reach the ear. This would be determined by several factors, but comes down to experimentation (i.e., try both ways and see what give better bass at the listening position).
Also, as noted, if they are mounted back-to-back (or front-to-front), this is called an iso-barik or compound arrangement, and you would want them phased in opposite directions so one pulls in while the other pushes out so they work together. If they are in phase they would both pull in or push out at the same time canceling each other.