Making sure both speakers are wired to the same polarity is about all you can do without using some series tools to measure and process the signals.
The 9V battery test check polarity, although sometimes we incorrectly call it phase.
The problem relates to variances in phase and group delay.
All speakers have some degree of variable phase shift. As the frequency goes down the phase lags behind. AFAIK, this is generally not a huge problem.
In ported cabs the phase tends to vary more quickly around the port tuning frequency. So if the cabs are tuned different, the phase variance between cabs will be different.
Often we discuss this problem as relating to mixing different driver sizes. But a good engineer can design 15', 12's, 10's etc so they play well together. If a pair of 410s are not designed to play well together it's crap shoot.
If both cabs have about the same sensitivity rating and power handling, it's likely that adding the second cab will increase the SPL. But it may also significantly degrade the sound. I have used severely mismatch cabs before, because I needed more volume. No one complained, but I did not think the sound was great.
Some argue that group delay variance is more of a factor than phase variance. I don't fully understand what group delay is, so I won't try to describe it other than to say it's a form of phase distortion, but not the same thing as phase variance. I do know that cabs that are tuned different will tend have different group delay characteristics, and if there is too much group delay variance between the cabs it can degrade the sound.
Most Eminence Pro woofers have cab tuning documents with several cab designs that show group delay.
Here is a link for the 2512 cab design doc:
290-593--eminence-deltalite-2512-cabinet-design.pdf (parts-express.com)
Here are some of the group delay graphs from the documents.
As you can see, some of the group delay curves are very similar. AFAIK, these tunings would probably be reasonably compatible. However, some of these group delay curves are probably too dissimilar to really work well together. Keep in mind all of these charts are for designs using the same 2512 driver.
My advice, is check the sensitivity, impedance, and power handling. Figure how the power will be shared between cabs and also between drivers. The idea is each cab must be able to safely handle all of the power you will throw at it.
Calculate the SPL each cab can produce with usable power, and if the SPL numbers are reasonably close, give it a try.
If the calculated SPL varies too much between the two cabs, adding the second cab may actually reduce the max volume your rig can produce, in addition to degrading the sound. So there is really no point in using both cabs.
