Basically we know twins are no safer than singles, but what you really need is a single and a twin doing exactly the same job in the same conditions to prove the case.
No, what you need is someone at the CAA (insert name of your regulator here) willing to sign off. I recall discussing single engine IFR with my FOI in the 1980s who agreed there was no reason not to allow, but then pointed out he wouldnt sign it off (on the theoretical basis that he could
) as he needed his pension and didnt want to risk the issues that might arise should an incident occur.
I thought this old argument had been done to death, resurrected and then murdered again.....