But the idea of two player games have been around for a while. One of the earliest examples that I remember running across was Paper Chase, an adventure for Call of Cthulhu who specifically designed for one keeper and one player. Heck, that one was in the third edition basic rule book.
I have heard it argued, quite convincingly, that horror games are best played with only one player. After all, everything is scarier when you are all alone. There is an entire Lovecraftian system called Macabre Tales designed for two-players and a noted collection of Call of Cthulhu adventures called Monophoboa.
And, over the last few years, I have found some interesting two player systems. Beast Hunters, Martian Colony, and Murderous Ghosts to name three very strong examples. Incidentally, all three of them have fairly tight structures for play.
Beast Hunters has the players swap the role of GM back and forth with a budget for threats. Murderous Ghosts actually uses a paired set of Choose Your Own Adventure style books for the mechanics, although you fill in all of the narrative details with your own imagination. Martian Colony's mechanical structure resembles a resource management, push your luck board game!
While there are some obvious benefits to only having two people involved in a game (like only having two schedules to worry about), there's a lot of issues as well. For instance, a lot of systems are designed to have a variety of abilities to tackle problems. (I cast magic missile/swing my sword/backstab the kobold! You kill it! And the other eight kobolds swarm you.)
Also, and this can definitely be the case in a Call of Cthulhu adventure, is that having only one brain working on a problem means the lone player can miss clues or other plot points. (Monophobia actually recommends using idea rolls to let the keeper flat out give clues player)
Two-player games also can have a stronger level of intimacy than group games. I've read more than a few which are actually about romance, with Emily Care Boss's Breaking the Ice being the one I'm the most interested in reading and researching. Whether or not this is a good or a bad thing depends on whose playing, I guess.
Two player games don't interest me as much as GM-free systems do. However, there's no denying there's a wide variety of them out there, both from a mechanical angle and thematic angle. It's a sub-genre of role playing games with some interesting ideas.