I find there is a fine line between a PC hiring a retainer and a player just playing two PCs.
This is probably due to reasons I've mentioned previously where I give the retainers to the players to play because I have more than enough on my GM plate. The players run the retainers, and for the most part they are fair. A lot of my players are either GMs themselves or are fair, level-headed people to begin with. When I hand out retainer NPCs to the players, I rarely have to take back anything the players says that the retainer does.
With this in mind, I'm wondering...wouldn't it just be better to let the player have two PCs? One can be the regular character, and one can be the lower-level character hired for a specific purpose (torch-bearer, loot-hauler, animal-caretaker, etc.) The player shouldn't expect a lot of the game tailored for the lower-level character to shine. However, the retainer is there, and it being controlled fairly, and is out of the GM's hands, leaving room for other things.