I'll weigh in with another perspective. It seems that much of this discussion assumes AL 80 tanks (as these are what is for rent everywhere). When my wife and I recreational deep (90' or below) we rent big tanks (100s or 120s). Thus, as individuals, we each are "carrying" a 19 or 40 cf "pony" With this built-in cushion, we have the same protection against going OOA as even the largest pony would give, without the need for extra gear, and manipulating it at depth or messing with it when gearing up. If there was an unexpected equipment failure for one of us, we would have the extra 20 or 40 cf cushion in the other's tank to get to the surface.
I recognize that big tanks are not available for rent everywhere, but where we dive it is our regular option.
This does not give redundancy, only extra capacity, but gear redundancy is what your buddy/team is for.
If I had to regularly deep dive with insta-buddies and could not get big tanks, I would get a 19 cf pony as a personal emergency reserve for any dive deeper than 90'.