I'm a 2 cab minimum guy. Interestingly, and this may surprise some people, the two cabs together can actually be as quiet as a single cab. Seriously! i have a thing called a volume knob on my amp, and I use that.
So 2 cabs can replicate 1, but can 1 cab replicate 2? Sometimes yes but in the case of the OP with 2 identical cabs, the answer is no. Like my friend in construction says "When in doubt, the big crane can do what the small one can, but not he other way around...".
Extra volume is only one part of it. For me it's about not pushing the drivers any harder than I need to. Not only does it keep the recones away, but it improves your tone by keeping you out of the region where power compression kicks in. I don't think there'd be too many people that would argue with the accepted wisdom to back it off if your speakers are distorting. IMO if you even get to the distortion stage you've gone too far because power compression should have already alerted you that the speakers are being pushed a bit harder than they'd like.