search for "matching cabs" or "mixed cabs" or "4x10 and 1x15" for endless hours of fun.
the basic principle is that for strong coherent low end you want all the same kind of bass cab; differences in phase response and "group delay" and other technical things mean that mismatched cabs run together might not sound as strong and "even" as identical sets of cabs of the same general output and quality.
PA companies don't mix different kinds of subs together for the same reason, they might not have as coherent a low end and might even "fight" each other at certain frequencies.
ideally, different models of cab from the same company will be designed to be close enough in behavior that they sound OK together, but that's not the same as being the best combination. if you want to guarantee a good result, find a cab that sounds good to you and if you want "more" get a second cab exactly like it.