In my world, I have an NPC (Baron) who made a deal with a devil (Stan) which requires Baron to maintain a diary in exchange for immortality. The many volumes of the diary serve to give Baron his immortality and also are leading to Stan taking dominion of the realm (unless, or maybe if, the players intervene).
I really dig the idea that reading a volume of the diary transports the reader into the memories of the author. "Reading" here includes the stipulation that when the book is read aloud, all within hearing distance get teleported into the memory. However, if the person holding the book chooses not to read out loud, or the entire party isn't within hearing distance, this scenario comes with the risk of the party being split.
My central fear is that the party is separated at the time the book gets read, and I have to find a way of effectively running two games simultaneously. Party splits aren't uncommon in D&D, since there are published dungeons with traps that split the party, but it's a difficult scenario to run.
What GM techniques or strategies can I use to run a game in which the party has been split for an extended length of time?