These are always interesting threads, and to some extent they boil down to four combinations, although 'shotguns' and 'muzzleloaders' complicate things - anyway - here's the pattern that comes out:
1. Centerfire Handgun, Centerfire Rifle
. a) Same cartridge
. b) Different cartridges
2. Centerfire Handgun, Rimfire Rifle
3. Rimfire Handgun, Centerfire Rifle
4. Rimfire Handgun, Rimfire Rifle
Rimfire/Rimfire doesn't make much sense to me, as you sacrifice power, and a really-good 22 LR handgun is close to as accurate as a 22 LR rifle, unless your 'niche' for the 22 LR handgun is a sub-compact gun, which isn't likely an "only two guns" goal, as even large 22 LR handguns are fairly concealable.
If there were a big need for CCW for self-defense against two-legs or bears, of course the Centerfire Handgun is vital.
If there were a big need for hunting or defense against really large game, of course the Centerfire Rifle is vital.
Keeping some sort of 22 LR is a good idea, because there is the potential to have a HUGE amount of ammunition portable, plus they are pretty quiet, even if unsuppressed, and suppression is fairly easy.
So what about a Centerfire Handgun AND a Centerfire Rifle...? That would sacrifice having at least one quiet, lots-of-ammo gun capable of small-game or quietly eliminating tactical 'problems'. When would that be worth the sacrifice...? I would submit that the primary reason would be if the Handgun and Rifle SHARED the same chambering, which means you could have lots of ammo (though not as much as a 22 LR) and supply both guns with the same inventory. A 'pistol caliber levergun' and companion handgun are perfect for this (though I guess so would an Encore pistol in 308 and a M1A). The only other scenario I could think of would be if there were a need for a Centerfire Handgun (bears, two-legs), but ALSO a need for a long-range rifle in one of the bottleneck cartridges like 223, or 308, or 30-30 or 45-70 or even 338 Lapua. That doesn't seem as likely other than if the scenario were 'urban survival', where you'd clearly be advantaged with an AR-15 or M1A or long-action levergun or whatever, AND wanted a snap-shootable handgun for surprises that came at you. BUT in that kind of crowded, highly-armed, environment, it would be likely that you could re-arm with any number of popular firearms if you weren't in the initial body count, and it might well be that having a quiet (suppressed) 22 LR pistol would serve you better in acquiring additional firearms than a noisier Centerfire Handgun.
So.....it seems like the most logical alternatives would be:
> 22 LR Pistol, Centerfire Rifle
> 22 LR Rifle, Centerfire Pistol
> Matching Pistol-caliber Guns
Those combinations were among the most common on this thread, which either means 'great minds think alike'. . . or. . . 'we are all doomed'....
I don't have any 22 LR rifles that can do all that much better than my 22 LR Ruger Mk-3, so the Centerfire Pistol, and 22 LR Rifle is probably out. That leaves either a 22 LR pistol with some sort of centerfire rifle, or a pair of 357 Mag/44 Mag/45 Colt firearms of some sort. The 357 is probably the best unless in bear country, because the guns are noticeably lighter, and so is the ammunition, plus it is more likely you can 'find' 38/357 ammo versus the other cartridges.
a) 357 Mag Marlin 1894 and 357 Mag handgun (.....hmmmm.....Desert Eagle or Speed Six
b) 22 LR Ruger Mk-3 and some sort of rifle. Probably an AR-15 since they are SO common (...but again, that means you could use your M1A, Enfield, BLR, or whatever to GET an AR-15