I am a complete newcomer to AD&D. I was wondering if anybody has had any experience with (as the title implies) one-on-one sessions. I was thinking about giving the player something like 20,000 EXP to distribute between two characters in whatever fashion they want and then turn them loose on some low-level modules. The point of this is to "warm up" and start fleshing out a setting prior to a full-fledged campaign (and for a fun rainy day activity). Re: the exp, my thoughts were that hypothetical set of two Heroes would still take a beating from say, 12 zombies (though of course they'd mow down a crowd of goblins or anything with less than 1 HD like a hot knife through butter) but it wouldn't be a hopeless situation. I am a newbie though and seek wisdom from those more wizened than me. Does AD&D hold up on this 1-1 scale? Have you had success with it? Please do tell.
If you're pondering how much XP to provide the PCs, Roger Moore did some good analysis of XP amounts by class in Dragon #69's "Charting the Classes"---worth looking up (also useful for building convention pregens too).
I've run a campaign for my two sons (who would usually bring between 2-5 PCs each into Castle Greyhawk), as well a solo urban and nautical trade-based campaign for my younger son Henry, in which he has 3 PCs, I have 1 DM PC, and they're running around with a shipful of merchant guards (and all work for the merchant). That's been fun, but it's not been dungeon-based, so semi-safer-ish so far (they've avoided a giant squid attack through dice ;) ).
Henry also runs a solo game for me, and I play 2 PCs, he has 2 DM PCs, and we have had some other NPCs in the mix from time to time. We almost had a TPK in his Scortop dungeon, but managed to not lose the party when we accidentally triggered a wave of humanoids attacking from nearby caves (a la the Caves of Chaos in spirit, seemingly). Half the party died, and all but 1 PC were KO'd, but we were able to escape in the end, and to raise the slain PCs after some travel to a larger nearby town.
Bringing more men-at-arms and henchman can help to balance out a smaller party, in addition to NPC adventurers (assuming they're reliable, unlike most of Gary's from B2 and T1 ;) ). Teaching the players to run away it even-more important a skill in a small party, though, so do that early! Multi-class characters can also help in this instance---although not with the HP totals....
Glad my inclination with multiple characters was correct. I didn't consider that with regards to saving throws. A carrion crawler or a grell becomes a truly horrid beast! I was thinking of things like forcing one character to be a fighter (if not a multi-class) for sweep abilities, giving one character psionic access (as long as they have Int/Wis/Cha above 16), and that high-ish starting EXP to distribute across characters to give them a fair shake. Maybe I'd run a level 1 henchman or two. However, gentle AD&D seems like not AD&D at all. Much to consider!
Semi-related, I recall vaguely reading an account of a session that was just sending a lone level 15-ish MU into the Caverns of Thracia - I don't plan on doing anything THAT outlandish!
AD&D is objectively bad at scaling down to one person parties. The system is designed to ruin lone PCs and require a large group that knows how to cooperate in order to beat the odds. Therefore although you can run a solo campaign with just one player, your inclination to allow them to characters is correct. Even so, you will probably have to remove most hazards that resort to saving throws since paralyzation fear and many other normally non-lethal threats to a party will completely destroy a group of one or two characters. Many of the other systems such as zero hit point, spell interruption, et.al. will likewise wreck a party of one or two. So you'll have to play very gentle.