Heresy; or, When Systems Collide

Something's been increasingly sticking in my craw lately. A heretical idea that refuses to die.

Why can't I run one system while the players play another?

I first ran across this idea in a long-forgotten blog or message board post. Someone talked about how they ran 3.5e for their players, but used Moldvay B/X behind the screen. In other words, the players got the use of all the feats and powers and whatever various other arcane complexities their hearts desired, while the DM jammed econo behind the screen, worrying only about basic hit dice, Armor Class, and monster-specific special abilities. Aside from adjusting AC from descending to ascending, there was very little the DM needed to do.

There's a peculiar philosophy in gaming that says that the GM has to be playing by the same rules as the players. On a certain level, this makes sense. I mean, no one wants to game with a GM who is making shit up as he goes along. But as long as the GM is operating within his own framework of rules, who says they have to be the same as the rules the players use? I mean, no one expects a DM to award experience points to his NPCs and keep track of their levels, for example.

I've heard it said that it's not unusual for even game designers to not use their own rules as written. But I've been thinking bigger. Why not use two completely different systems?

The example that got me to write this post comes courtesy of an old post on Risus Monkey in which he talks about using Risus as a shorthand for GURPS NPCs. One of the things that drove me away from GURPS, as much as I admire the changes implemented in Fourth Edition, was the d20-esque stat blocks, with their line after line of different abilities, exceptions, and special powers. As Christian aptly puts it in a comment to this post (which features a fine example of the sort of stat block I'm talking about), GURPS character creation is almost a mini-game unto itself. Now, that would be highly desirable to me as a player. But as a GM? A shorthand system for creating NPCs is a godsend.

But why not take it a step further? With a Rosetta Stone like the one Risus Monkey posted, I could easily run a Risus game while my players played GURPS. Any interactions between the two systems could be quickly solved using the Rosetta Stone. Obviously this would work best (or at all) with a complex system and a simple system, or two simple systems. But that's the only reason I'd do this anyway; my priorities as GM these days are simplicity and speed, both for prep and for running a game. BRP and Savage Worlds meet those needs well enough, but there's just something about more complex systems like 3.5/Pathfinder or GURPS that I think are super fun for the players, and I hate to deprive my players of that opportunity should they choose to so indulge.

At any rate, these are all just random thoughts at this point. I don't anticipate a chance to put my heresy into practice any time in the foreseeable future. How about you all? Has anyone tried running one system while their group played another?