..merging the two must have happened across the whole of Starfleet, and not just on the Enterprise. In Voyager Tuvok was both Security Chief and Tactical officer.
That doesn't convince me.
Voyager was but one ship, not a representative example of Starfleet at large. And she was a ship making do with replacement personnel after suffering major casualties. Tuvok only assumed the combined Tactical and Security position after those casualties - until then, he was spoken of as the Security Chief, off to an assignment.
Really, those casualties were later said to have included at least one unseen Commander and two Lieutenant Commanders in addition to the XO, LtCmdr Cavit. One of those LtCmdrs might have been the Chief Medical Officer, but we would need other top positions to accommodate at least one LtCmdr and one Cmdr, and preferably the latter should be out of the chain of command since he or she outranked Cavit yet Cavit held the highest position in the chain of command below Janeway...
With Security already taken by Tuvok, the remaining LtCmdr could well be Tactical. Or then Chief of Ops, another known casualty because his or her loss allowed Ensign Kim to take over that position. Or Chief Engineer, who also was lost. And the mysterious Commander could be Chief Science Officer, to keep him or her out of the chain of command.