One relay for low speed and one for high speed. The controller shuts the low off when the high comes on and vice versa. If you wanted to control them normally, you'd actually have to wire them in such a way that both relays wouldn't be closed at the same time.
The fans draw about 20 amps constant on low and 40-45 on high. Since most other relays are either 30 or 40 amps, you need 70 amp ones to be reliable. In the contour, they use 2 70A relays and a 60 Amp Maxi-Fuse. I just copied what they did.
Its likely the inrush from stopped to high is near 70 amps for an instant. This won't blow a 60A fuse, but its a lot of current draw and would likely fry a 40 amp relay pretty quick.
