I’d agree that you’d be better with two fans in operation as when the stage 2 fan engages as that’s the idea, one fan isn’t enough as the temp has gone so the rad sender engages the second stage, going from one fan to the other won’t improve your cooling one bit.
It’s a nice little project sorting the fans and wiring out.
Something else to bear in mind is the auxiliary water pump, you need to see how he’s wired the fans. If he’s bypassed the main fan controller unit chances are this won’t be running if the fans come on when you turn the engine off. You can tell if the auxiliary pump is working by just turning the ignition on without engaging the starter motor, you should hear the pump run for 30 seconds or so, no buzzing and it is duff.