Wiring a speaker out of phase with sub connected

Probably a dumb question but here goes:

I changed the X-over frequency on my subs and want to re-adjust the phase to make sure they are good. My subs get their signalk from the 2nd set of binding posts on the main speakers.

Someone on this forum learned me a way to do the phase by reversing the phase on the main and then playing a test tone at the x-over frequency and adjusting the phase until its as close to canceling out as possible. And this worked GREAT. But now with my subs running off the mains for a signal I am unsure how to approach this.

My thought is (and don't laugh) is the reverse the phase on the main and then reverse again on the sub so the sub is the opposite phase of the main.

Would that work? I only know enough to burn my apartment down with this but I feel like this would work (getting the main and sub opposite [phase from one another]