I much prefer a lower temp fan switch, or just a manual override switch.
Still, if it were done properly, either brake switch could activate it, because the front switch is in parallel with the rear switch. However, without really diving into it, I think you will need to add an additional switch to activate the fans, because the existing brake switch connects power to the light bulb - the bulb being the load. The bulb has a hardwired ground connection.
Whereas, with the fans, the load is always connected to power , and the switch connects the load to chassis ground.
A relay would likely be the best option.
Let us know how it goes if you do it.