So it seems to me that if multiple tanks is your answer to the rec dive redundancy issue, and you're traveling by air and want the convenience of renting tanks, then sidemount is your answer.
Sidemounting the ubiquitous Al 80 seems almost as convenient to me as a pony if you're already looking at a 40cf pony. For all I know, it's actually easier in the water than having a slung pony.
I am in the camp that uses my buddy for redundancy rather than a pony, but if I were to instead decide that my redundancy will come from having two tanks, I'm certain I would be a sidemounter and use sidemount on every dive, whether at home or in a distant location, so long as the dive op doesn't object. Indeed, if all of us demanded sidemount for redundancy, dive ops would be forced to cater to it. If redundancy on a rec dive is as critical as we say it is, then maybe the standard configuration that should be taught in OW by all the agencies should be sidemount?
But back to today's reality--and this goes back the nagging question I had when I last participated in this thread pages ago: If you feel you a need for two tanks, then why not face up to the fact and just consistently dive two identical tanks in a conventional sidemount (or backmount) configuration, using the well-established procedures that go along with those configurations, rather than trying to couch the other tank as a "pony"? (The mention in a previous post of an 80cf "pony" is what caught my attention and prompted me to chime in again.) I am in the camp that values consistency and proven procedures, though I noted the comments upthread about consistency, muscle memory, etc. Sidemounting would address all that.