Following suggestions from users on here, a bit of patience and allowing Tempest to be in full noon sun for several days resulted in settling of UV measures.
Sky and Tempest are now closer for UV than they were at the start (delta 0.5 vs 5.0).
So, patience sometimes helps 
Indeed, Sky and Tempest received a calibration yesterday at the same time. One up and one down …
Happy they agree finally 
Just like magic… it evens out :). Thank you. They’re not identical but in the general ballpark now.
I also noticed that Sky and Tempest are showing pretty much the same wind direction now, so not sure if that got any kind of nudge too. I’ll have to wait until there is zero wind to see if Tempest insists there is wind out there.