Answering just from your thread tittle (without reading your actual, whole question:
Just by virtue of the fact that people would be looking at TWO amp heads instead of one I'm sure that they would APPEAR to be louder!!! (I was waiting for someone else to be the smart-ass, but......)
Ha! You know that's right! Purely psychological; plenty of 'big-time' guitarists(Rick Nielson in mind)have a wall of dummy 4x12" cabs on stage, while only playing thru a mic'd up small Fender or whatnot. A friend of mine who is a pro sound guy told me Robin Trower currently does the same thing. But realistically, most of the audience wouldn't know or care.
A lot of times the faux stacks are made of balsa wood and fabric, and I think way more often than not if you see a “wall of amps” it’s a stage prop rather than functional gear. In fact, your comment reminds me of a story I heard about actual size blown up
pictures of amps going on a European tour instead of the prop rigs…
It’s actually kind of pointless to stack up a bunch of MI gear nowadays, modern PA systems are so much more efficient and effective for delivering LOUD than MI stage gear that it’s not even close. In the olden days it was necessary; Mountain’s Sunn walls, Grand Funk’s West Fillmores, Noel Redding’s 3 2000S rigs, etc. were an integral part of getting the “Experience” to the crowd.
Of course bass players can still get away with bigger rigs and more stage volume because they are down low, not that obnoxious high/high mid guitar register. To refer back to your Rick Nielsen comment; on the
other side of that stage Tom Petersson’s rig is mostly all plugged in and functioning.
Today’s high powered line arrays can get
stupid loud. In order to keep adequate headroom most pro SR systems have the capacity to not only well exceed any venues dB restrictions but to also get to an instant, permanent inner ear damaging level of “loud”.
In a pro gig situation once you get your tone generally the best bet is to let the SR system do the heavy lifting.
Getting back to the original question, I hadn't seen the topic discussed before, and was very curious from a technical standpoint. I've previously owned a 2000s, used it on gigs thru a 2000s cab AND a modern 4x10, both were pretty forceful but had different tonalities of course. However, as I stated before, I'd rather lift a 200s style head twice, far more manageable, than sling around a single 2000s. And EdBass, I hear you, but I just wonder if the 'bigness' you refer to is a byproduct of the speaker cab size, or brand of the drivers within.
Even with identical cabs I think you get a presence with one big amp that you don’t with two smaller ones.
99% of the time I use one or two of these 215S cabs;
loaded with EVM15Ls. If I’m using Sunn power
only, sometimes I’ll replace them with one or two D or K JBL loaded 200S cabs, but most of the time I use a Reeves as primary and I don’t want to get too enthusiastic and pop one of the vintage JBLs, so I generally only use the vintage stuff on special occasions.
Anyway, I’ve used these cabs with both a single 2000S as well as with a pair of 60 watters, and while tonally as well as visually the pair of amps “wins”, the single 2000S seems to give me more thump and headroom. Of course another issue with dual amps as far as maximum volume is phase issues; as I posted earlier IMO getting two vintage amps to truthfully perform identically is pretty unlikely, and if they are out of phase even a little it could compromise the ultimate SPL potential. Not very
much probably, but a single amp wouldn’t have that issue. Tonally, sometimes multiple amps can be a good thing; a little “thickening” effect when you blend the two, but IMO regarding your initial “louder” based post it could be counterproductive.
I’m the opposite of you regarding convenience; I would rather make one heavy trip as opposed to two lighter ones. My only problem with using a 2000S is the awkward size of the head cab rather than the weight. Most of my cabs are 24” wide and to me that 30” wide 2000S amp looks totally lame on top of one, I have such an issue with the aesthetics that unless I am using a 2000S cab (which is almost never) I put the amp on a stand instead of on top of the cab.
I recently picked up a Sorado cab(one piece baffle with two smaller tube port holes)with a couple of old Altec 421's; while it did have a certain low-mid punch, it sounds quite different than my JBL/200s cab with the classic front-loaded horn structure--more low end 'whoomp'. I just snagged one of those whacky 3x12" cabs with the angled baffle sides; unfortunately none of the three speakers matched, but when I can afford three identical drivers that can handle the bass, I'll report on it. I'll probably use it in a 'hillybilly bi-amp' fashion, powered by one of my 60 watters with some of the low end rolled off. Slowly approaching the Entwistle rig, as i tell my girlfriend, I'm not a Sunn hoarder, I'm a Sunn curator. 
IMO those 200S cabs freekin’ thump down low, I imagine that mid range punch you describe is probably as much bottom end attenuation as it is any low mid boost compared to a 200S cab.
I’m lucky on the significant other issue. My wife doesn’t really care as long as I don’t impose on “her” house (which of course I do), the bills get paid, and the
joint bank accounts continue to grow rather than get depleted by gear expenses.
