We have found a key ingredient in double casting is structuring the rehearsal time. It is an advantage to having a "gymatorium": cast A on stage acting full out while cast 1 is in the gym going through the motions but quietly reciting lines. I've also had the doubles or understudy shadow the other just upstage. Again, the focus is giving both enough rehearsal time beyond simply watching and listening.
I love the idea of the short pre-show piece! We produce a One-act Festival between our fall play and spring musical. It often includes student written work (our Creative Writing Club) and student directors (seniors who have been in the program for 3 years). It is a great way for new students to try acting (lots of walk-on opportunities) and for more students to have lead roles. We keep everything simple: set, props, costumes and focus on the acting and usually rehearse 1 or 2 hours a week.
Peace and blessings,