Why are there two safety cars and two medical cars at every Formula 1 race?
Bernd Mayländer only drives one of them...
Medical Cars and Safety Cars have different functions in Formula 1.
Based around the circuit, mobile response teams include four salvage cars (S-cars) and two rescue cars (R-cars) as well as two extrication teams. The S-cars are equipped with a rescue cutter and fire extinguishing agents and, if necessary in an emergency, are able to tow a damaged car. They are manned by two experienced helpers. The R-cars are manned by an emergency doctor, four paramedics and a driver. They can reach any point on the track within 30 seconds.
Source: Formula 1: Medical
The safety car comes into use during a race when the Race Director wants to reduce speed for safety reasons - for instance, after an accident or because the track is waterlogged after heavy rain. According to the regulations, the safety car enters the circuit “whenever there is an immediate hazard but the conditions do not require the race to be interrupted”.
The safety car is on standby throughout a Grand Prix, ready to be dispatched by Race Control at a moment's notice.
Source: Formula 1: The Safety Car
The official driver of the safety car is Bernd Mayländer, and he's only in one car. If there is a second safety car at each race, I'm sure it's only there as a backup in case of a breakdown of the primary safety car.
Although the previous post is very informative I feel there's a bit more information available.
In the late 70's when Bernie was transforming the sport, improving revenue and safety - taking the power (and profit) away from the circuit owners, part of his 'negotiating' strategy included the insistence of 2 medical helicopters at every race.
If this wasn't provided then the teams would boycott the race. He demanded that a helicopter was available at all times, and if the helicopter was taking a driver to the hospital (and there had only been one) it would have left none at the track - and so practice/qualifying/race would be suspended (no good for the tv coverage - which had really taken off after the 1976 season, track etc). And so a second helicopter was required to cover the first.
As far as safety and medical cars go, I can imagine that the need for 2 is partly an extension of this - if something happens to one the show must go on. It would be extremely embarrassing for such a high profile sport to be delayed whilst they fix the medical car (for example).
Imagine the World Cup Final delayed because someone has just kicked the (only) football out the ground and one of the ball boys has to go and fetch it (fish it out the Trent e.g ).