Pont 1. Two cabs usually sound better than one cab because louder is always bigger.
Point 2. It's possible for mainstream manufacturers to investigate the phase response of all their speakers and come up with cabs that phase together correctly across the whole bass range, but no mean feat.
Two Genz cabs are claimed by a Genz engineer on here to always sound great together. Presumeably only cabs of concurrent design series carry this claim.
When you look at some of the design howlers that make it into production from some mainstream manufacturers you kinda doubt they looked into phase response at all, probably due to point 1.
Point 3, any two identical cabs give you more of what you like, no question.
Point 4, any two non matched cabs may sound ok together.
Point 5, lots of guys mix cabs and like the results, like to tell the world about it.
Point 6, lots of guys stopped mixing cabs, got better results and like to tell the world about it.