Linda, our club usually has two rings, with two judges, to handle more entries.
But I've been to trials that have two rings with one judge. While courses are being run in one ring, the other ring is open for course builders to set the next course. Then that ring will be run, while the first one is changed to the next class. Of course, that does mean having enough equipment to do this, but if you did, say, tunnelers in one while jumpers was running, then the tunnelers changed to regular (if you have enough jumps), etc... basically setting up the run order to not need to use contacts in both rings if possible (unless you do have enough to duplicate in the rings), that will save a lot of time as the trial can continue running while the next class course is being set.
Hope that made sense!
Jessi