Right now, my plan is to completely finish Darrell & Chester's routes before moving on to the next "batch".
For each new build, I'll alternate between the two. I'm thinking (not set in stone) I'll make Build 4 be about Chester, then Build 5 will be for Darrell, and so on.
All-in-all, it'd be 6-8 builds to complete both Chester & Darrell then move on to the next two.