My last D&D session ended with the party split three ways. Usually I try to discourage that sort of thing but they're up to some sneak subterfuge and it made sense to let it happen. However, I did call the session early because I simply didn't feel prepared to deal with that kind of complexity.
In preparation for the next session, I put together a one-page procedure for dealing with parties acting in multiple areas simultaneously. The idea was to figure out a consistent way of dealing with these situations without having to do too much handwaving in terms of timing or positioning. The full procedure is below the break. It fits nicely on a single page of Letter or A4 paper.
N.B. This procedure assumes a) an OD&D-style turn structure where one turn represents about ten minutes; b) fairly structured social interaction procedures as per Courtney Campbell's On the Non-Player Character.