Yeah, most people only have one personal secretary, and in order to be able to show favoritism to one employee over another it would be necessary for them to be in similar positions. It would be remarkably silly to say "You're giving me the tough coding assignments instead of giving them to your secretary, you're blatantly favoring her" when the secretary was never hired for her coding skills.

Favoritism certainly can happen in other situations, of course. Heck, "the GM's girlfriend" is a (negative) trope in tabletop RPG groups for a reason, or spouses favoring each other in competitive board games. And those are situations where (usually) no money is involved, and it's just for fun. Add money to the situation and it can be a dangerous mix. If someone really must end up being a supervisor over his own spouse plus other people, best to have someone neutral double-checking his decisions. It's easy to be blind to one's own conflicts of interest.