Someone said a 220 and a 226 with a 22 conversion kit.
How about a 250 with different frames, so one pistol is full size, compact, and sub compact. Oh yeah -- also different calibers, if you feel you need that. So, you have what is officially one pistol with different sets of "clothes".
Second pistol a Glock 17. Then a couple of pistol caliber carbines, and some revolvers.
TWO pistols - problem solved. If I were to cheat and get a third pistol -- we are looking at a good 1911.
Good question - I approached this by looking at my modest collection and thinking what 2 would I keep if I had to sell everything else off (a very real possibility given my current financial situation). The two I would keep would be my Series 70 Colt Govt model .45 and my S&W 586 .357 L-frame. Lots of years on those guns, fond memories, and I shoot them very well.