My definition of " do my best" is best on the average. Given that we know no two sources that are not co-located in space are going to have the same arrival times at different points in the audience, I have to wonder at the constant "at what point " comment. Because we know it is not possible do we stop trying for as good as possible, give up and go home? Give every audience member headphones?
Given the range of differences in arrival times that result in coupling at sub wavelengths, we do have quite a bit of practical leeway in setting up that gives decent average coverage.
Everything is about compromise. I think my models show that combining the subs may possibly (depending on their difference in phase response) give good results over the listening area at one frequency. If the phase responses differ greatly, all bets are off at other frequencies / locations. That is the compromise, hence my suggestion of limiting what is in both systems to what is benefited by strong response at that frequency.