Secondly the missus is incredibly concerned about flying in a single engine plane, which also means she's terrified about me doing it!
What might help (or hinder) you to convince the missus is that the statistics show very little safety benefit from a second engine, as long as we are talking light twins here (not airliners).
Reasons for this are very complex, and statistics can of course be manipulated any way you want, but here's the summary of how I see it:
The technical safety benefits from the second engine are relatively small since climb performance on one engine is marginal at best in low end twins, and may be completely nonexistent in unfavourable conditions. So the second engine is only taking you to the site of the crash in any case. (And with two engines there's twice that many bits that can go wrong...)
And the few technical safety benefits there are, are typically negated because pilots take twins on more dangerous missions, like IFR/night, high altitude or long overwater flights.
(Cue a long debate on technical and statistical safety benefit of twins!)