Correct, but that's the low load side of the equation.
This is similar to using two relays and independent feed circuits for hi/low headlights systems. If you run one power line and that fuse pops, feeding both hi and low, you have no lights. Running two circuits, if the low side pops a fuse, you still have the hi beam side as a fail safe to get stopped. Run one power feed to both fans and if that circuit fails, you have no fans. 50% of something is better than 100% of nothing.
You could always put a rocker switch to bypass the temp sensing switching system too. That way if the sensor fails, you could toggle/rocker switch it to activate the fans. A little redundancy.
