The resistor controls low speed on both fans. High fans come on bypassing resistor at 240+ degrees or around 250+ psi highside on a/c.
Car will run with high fan only but it's a backup, you'll toast engine if that fails, the double fan speed setup is an automatically backed up system unless you insist on running it half failed. Head gasket jobs are always cheaper than resistors right?
When my resistor failed I rebuilt it and connector for like $7-$8 worth of parts. The resistor coil itself does not go bad, only the connections.