How to run two adventures at the same time and totally screw with your players. An idea looted from
Ravenloft II: House on Gryphon HIll by Tracy and Laura Hickman
Do you have to adventures you want to run but only one group? Well it is time to mind screw your players by running them both! These adventures can be on different worlds, or perhaps separated by different epochs, you can cause them to be mystically linked to each other though the Player Characters.
The way PCs cross this bridge is through the strange and mystical delirium episodes they suffer. When struck down by this ailment, the PCs awake to find themselves in a different land, faced by new challenges. Your players may begin in the 1st adventure, fall into a fever, and awake to find themselves in the 2nd! Then, after adventuring for a time, they suffer a relapse and regain their senses in the 1st adventure. There, a Non-Player Character tells them how they lay in a coma for several hours, babbling and murmuring. But, when they once again return to the 2nd adventure, a different frightened NPC there tells them the same thing! The PCs will never know which world is the dream and which real.
That is one of the decisions you as GM must make. You may make one of three choices. That the 1st adventure is real and the 2nd adventure is a fevered dream; that the 2nd adventure is real and the 1st adventure is not; or that both adventures are parallel and equally real (this is the one I recommend. In the dream world, the normal rules still apply. However, I recommend a linked villain that must be defeated in both adventures, if the characters defeat the villain; it will have no effect on the other adventure and vice versa. If the villain is not defeated in both worlds, he will rise from the dead or have fled to another form when the PCs return. However, if both worlds are real, then the villain, like the PCs is crossing back and forth. If he is defeated in one world, he is defeated in both. This gives the PCs more opportunities to defeat him. It also gives him more opportunities to defeat them, too!
While in either world, the PCs can suffer damage and die. However just like the villain, if you have to die in both worlds to suffer a permanent death. Otherwise, upon entering the adventure where the PC died, the PC is alive and well. Hit points lost are regained each time the PCs move from one adventure to the other. The time spent in a fever does count as rest, thus characters can regain spells and abilities, plus heal other damage normally. Items gained in one world will not be transferred to the other, with the exception of the character’s starting equipment.