It's vaguely been discussed before, and is not "unheard" of. Houses that had a second story had alternate paths for individuals and their entourages who were unable to climb them. Run, famously, had multiple paths where scareactors could close doors and block force people a different way (to emphasize the "trapped" feeling). They stopped it after one weekend. Screamhouse had an "alternate room" that would get people so confused they would walk through the same two rooms for fifteen minutes (no joke). For you veterans who remember Screamhouse, the "basement" allowed guests to turn left and stay in the dark, or go straight into a red-strobed "Bondage Room" (so name because of the costume). They would walk out of that room and, instead of turning right to continue with the house, would just keep turning left in a massive circle. As they went the wrong way, they would convince guests who were correct to turn around and get everyone stuck. The closed off the second room during the second weekend. There are just too many "guests issues" with using multiple paths.
The Conga-Line: I have watched guests, while following the person in front of them, walk around in a circle, or go straight through a boo-hole because all they are doing is following the person in front of them. It happens far more often then you think. You see the same mentality when loading on the Mummy (where it branches right before the loading platform), when one side will be completely empty and everyone else is just standing in line on one side. Even with a Ride Attendant telling people to use both sides, they just stand there. That problem will be exacerbated in a dark haunted house where, for many guests, the person directly in front of them is a form of security. This would lead to long down times for scareacters who are just standing in their boo-hole, waiting for someone to "choose" their path. That ruins the rhythm of the scare and makes for a long, not fun night.
Indecisiveness: In the Coughin' Room in Catacombs last year, numerous people (at least twice a night) would stop dead in their tracks because they didn't know which "way" to go. This was with two scareacters in the only two other "paths" and an op waving a flashlight towards the exit of the room. It stopped the entire house because people couldn't figure out how to walk forward. Give people an actual choice and that wait will become longer... unless...
The Solution: The best way to prevent the above problems is to not give the guests the choice. This can be done by changing the path with each set and the Ops perform a "cut off" depending on which Cast is on set, or have a Scareactor who forces guests the way they can go (which can be done by literally shutting the door in their face and breaking the congo-line). The problem remains, however, in how guests will act. Returning to last year, guests would literally try pushing past us to get through our boo-holes because they thought that was the way they were suppose to go. A scareactor shutting a door in front of someone is going to cause problems, because the guest will either get angry and confront the scareactor or freak out and refuse to move. Having an Op do it isn't much better, plus it takes away from any atmosphere the effect is suppose to achieve.
Plus the fact that if you block one entrance, you have to block that path's exit as well so guest to try to turn in that direction. If you have to pay for multiple scareactors, equipment and props for multiple paths, etc. It is possible, but I, truly, don't expect to see it any time soon.