I have a similar situation when we have plenty of people around + different cooking temps for different foods + not enough grill space to do everything. This in itself can be a challenge to decide what first and how, but the more experience I get the better im able to cope...and the more efficient I become (still a long way to go).
Last time I decided to use the hotplate I have had in the loft for a few years so i could keep food warm outside, this worked extremely well as I was able to get quite a few veggies done before time to eat, and then focus else where. It also allowed me to take my time and not panic....which made me more organised and efficient.
I usually plan exactly what order i will cook in, plan on how I will change cooking configurations and plan to get food served 30-60 mins early. A cool box allows your low and slow to finish quite abit earlier, allowing time to crank the heat up(since time seems to run away).
if im doing a low n slow cook (or a roast) indirectly then i like to choose sides that can be cooked at the same time and just added later into the cook. I try to get a nice choice of food served at the same time before changing to direct.
Your suggestion of having 2 grills is something that I am considering myself. 2 grills at different temps would certainly be a great team.