It is very hard to run live theater back to back. First off it would be murder on the cast and crew secondly that is small potatoes as you could theorecticly double cast and double crew the show. The cost would be extraordinary but ignoring that it often is just not practically possible. In just about every show I have worked on crew call is somewhere between 1.5 and 3 hours before a performance and cast call is between 1 and 2 hours. This of course depends on any number of factors not the least being makeup and props. However most shows also run a dimmer check, mic checks, rigging checks, and safety sweeps. Further you have to have time to fix things. If a lamp burns out on a instrument it takes time to bring out a ladder, find the right replacement lamp, change out said lamp, and validate instrument focus.
Live theater is just that, LIVE. You have to have contingency time. I tell people that during the run, not the build up to but during the run, every minute on stage is between 4 and 6 minutes of prep and tear down.