I can't see any reasons for doing it separately for the tank and for the fish.
Recommendations (arguable) for the pace of saltifying are pretty much the same for the tank bacteria and for the fish, so I'd just do it in one tank...
Would do and have done, that is.
Acclimating fish in a separate tank makes sense if you already have an established brackish tank and you want to introduce fish that come from a different level of salinity.
Doing it slowly makes a lot of sense, in any case.
Good luck!