Just got this delivered today in the UK (John Lewis) I also have a fenix 5 plus (3HR before that), I tend to only wear the fenix when I do activities but have been wearing a fitbit alta HR on my other wrist so I can wear classic watches mix/match but get all day steps and calorie estimates.
Ive now switched my fitbit to the vivosmart 4 as my "all day device" I noticed that if I select it as my preferred device (tip says worn most often) if I run and set the fenix 5 plus to record it, the vivosmart kicks in and also record a run at the same time, it seems that true-up or the fact its preferred tracker means the fenix 5+ data is not uploaded or discarded/ignored even though it has more data (GPS, cadence, vertical oscillation etc etc) any way I can make my all day scenario with the vivosmart work but have it prefer the fenix 5 for activities.