That raises an excellent point. During the last ammo shortage under Obama, you couldn't find 9mm at Walmart (where I lived, at least), but you could often find .40 S&W. During that same period, Walmart tended to have zero .223 or .308 on the shelves, but did have 6.8 SPC, .300 Blackout, and steel-cased 7.62x39. So, one possible approach is to have a handgun that will fire 9mm OR .40 S&W, and a rifle that can handle a NATO cartridge OR an alternative. It's often as simple as swapping the top half of the firearm (and maybe mags), and the additional parts usually cost less than buying a second gun...
I think in a post SHTF scenario, we're going to best be served by having firearms and ammo setups that are simple, rugged, and reliable. While zombies are still infiltrating your perimeter every day, it would behoove you to have as few complications between you and the bang switch as possible.
This is why I own multiple weapons of the same caliber, and it's why I keep those calibers restricted to the most common varieties around.