I remember one having to use both controllers to save the game. Was there some viable reason for this or is it a 'how did they design it that way' CP?
The Famicom (Japanese NES) has two hard-wired controllers, so it was not uncommon to find single player games that used both of these. The second controller had a mic, and lacked the "start" and "select" buttons.
In fact, the Japanese version of The Legend of Zelda already used the second controller in another way. Pols Voices were weak against noise - and in the Japanese version, you killed them most efficiently by blowing into the mic on the second controller.
This "save menu" trick was even listed in the Japanese and US manuals for the game, so it wasn't a secret or a leftover debug item, it was a supported feature of the game.
From what I remember reading, the early designs for battery-backed up game cartridges for the NES were pretty weird. Later NES games didn't require the player to use the second controller or hold Reset to save a game.
The thing to remember about many NES games, they often had their own special processors or other special hardware in the cartridges to extend the functionality of the NES. In this case, that's just how the memory controller that managed the game save worked.
I'm rather certain that there were no technical limitations. They were probably just trying to reduce the chances of accidentally pressing the combination. If you didn't have a buddy who was using the second controller (generally uselessly), then interrupting your game (by bringing it to the save menu) would have been rather challenging to do by accident.
Another example: Metroid supported holding A and up on the second controller, while paused (similar to Zelda).
Side note: Mega Man 3, NES release also supported interaction with the second controller. (While holding Right, Megaman could jump high, jump out of fatal pits, and sometimes there would be other effects... I believe if MegaMan got hit, or perhaps some robot masters, then there could be some effect.)
Of course, you could also save the game just by dying. Until I read about the A+Up-on-second-controller in the manual, that's how I saved.
Correct me if I'm wrong, but LOZ for the NES was either the 1st NES game to feature saving, if not 1 of the first. Developers wanted you to be able to save your progress and end the game at any time. So the Up and select on the item screen was a way to do so without complex menus, and the shortcut wasn't one you would use by accident.