I have never done it and don't know much about its execution, really, but it might be valuable if you spend a lot if time idling in the heat!
Some vague considerations:
1) Engine-driven fans move a lot of air so make sure you arent actually restricting maximum airflow when the primary fan kicks on.
2) If it is switched, you might make it come on well before the engine-driven fan, rather than at the same time, or it might actually cause the primary fan to have to kick on more often than without (even if together they can bring it back down faster, it might get hotter more quickly with the unpowered fan blocking some airflow).
Maybe even make it manually switchable, and just leave it on any time you are doing a lot of slow-speed driving or idling in warmish weather.