Hi Velke,
the dual time is in my opinion just a way to have 2 different clocks on your watch, ie with a different time zone. I have for example my own time (the netherlands) en as 2nd time the time of USA. But i use it almost never.
I don't know exactly how important the activity setting is. Do you use it in
training manager? (I have the
T6, don't know exactly the differences). I don't give much attention to it, but the
training manager calculates it by itself if I upload some logs.