In 4 years with our S P85, and over 82K miles - we've really only needed to do charging at 80A once - and that was because there weren't any superchargers at that time, and we needed to have a full charge before making a road trip.
We didn't order the 72A upgrade for our new S 100D - and should be fine with the 48A charging at home. We'll use the HPWC at 48A for the new S 100D - and for our dual charger S P85, we should be fine with running it off a 14-50 at 40A.
And, if we ever really needed to fast charge the S P85, we could swap the cars in the garage, though that seems highly unlikely.
When we can place the order for our Model 3 (to replace the S P85), we'll likely stick with the standard charger - and plan to charge at home at only 40A.
Plus, the Model 3's will be lighter - with smaller battery packs getting longer range - so there'll be even less need to have high speed charging (which might not even be an option for the Model 3).
I'd recommend doing what we did - if you have an 80A HPWC - plan to use that at the maximum setting for one car. And for the second, use a 14-50 outlet with Tesla's 14-50 cable (so you don't tie up a UMC) - which is much cheaper to have installed. And if you don't have enough amps to have both an 80A HPWC and 40A 14-50 running at the same time, then throttle down the HPWC so you can have both cars charging at the same time.