Single runway major airports are no more "dangerous" than multi-runway airports as long as procedures are adhered to. The main reasons for having more than one runway is capacity and business continuity, although how much capacity is gained will depend on the separation between the runways and their alignment. For example if used in segregated mode (one for departures, one for arrivals) and aircraft need to cross one runway to/from parking areas then this will affect capacity on the runway to be crossed.
Gatwick's northern runway was constructed purely for business continuity, so that if the main runway was closed for planned maintenance or blocked due to an unplanned disabled aircraft then they could continue to operate although not to the same capacity level as the main runway. The depletion of Fire Service cover in the event of an aircraft emergency or accident can delay the re-start of operations but at very busy airports with two runways or more which have more than one fire station there is the option to provided the appropriate level of fire cover at both so when one responds to the incident the other has the cover available allowing operations to re-start very quickly, although this would depend on the seriousness of the incident.