You can join threads elsewhere within reason. Fluid time currently gives you between 2 and 4 months (currently 4) to work with. Every thread you do needs to be Timestamped (=you put it on a calender when it happens). If you simply take into account the time needed to travel between two cities in your timestamp then you can create a thread said in another city.
The
Simple Timestamps link from my sig could be useful with helping you find out what months are available. Alternatively you can use the announcement on what season we are in on top of each city forum.