According to
this analysis of Seneca fatal accidents, engine failure accounts for 11.3% of them.
As it looks only at the Seneca, it's based on just a small number of accidents. The overall picture for light twins is significantly better than than, but still not stellar.
Mike Busch wrote an interesting article for AvWeb which takes an even-handed look at the statistics:
The overall accident rates of high-performance singles (like Bonanzas or 210s or Mooneys) and light twins (like Aerostars or Barons or Commanders or Cessna 310s) are astonishingly close. Twins have a slightly higher accident rate per 100 aircraft and a slightly lower accident rate per 100,000 hours, but for all practical purposes the accident rates are the same. The same is true if you consider only "serious" accidents that involve death, serious injury, or substantial damage.
[...]
The statistics showed that a light twin is about equally likely to have a mechanical-caused accident as a high-performance single. But the twin's mechanical problem is most likely to be gear-related while the single's is most likely to be engine/prop-related. A single is about two-and-a-half times more likely to have an accident due to engine/prop failure than a twin (8% versus 3%). And if we assume that a twin is twice as likely to have an engine/prop failure (since it has twice as many to fail), then we can conclude that an engine/prop failure in a single is five times more likely to result in an accident than an engine/prop failure in a twin.
So are you any safer flying a light twin than a high-performance single? In terms of the overall and serious accident rates, the answer seems clearly to be no. But your risk profile changes somewhat: in the twin, you're less likely to be hurt by an engine failure, and more likely to be victimized by something else.
I always find it interesting that people concentrate on the supposed safety benefit of a second engine when the statistics show an almost negligible benefit. It's an emotional argument, rather than based on reality. If what they were actually interested in was reducing risks, they would be looking at the one factor which the statistics clearly show provides a safety benefit. That's not a second engine, it's a second pilot. Of course, it's a lot easier on the ego to believe the engine is the weak link in the chain, rather than oneself.