Exactamundo, controller.
Clearly it is a "given" that twins are safer than singles, "everyone knows that", its "obvious".
Isn't it?
If the complxity of an aircraft increases as the exponential of the number of engines and the vast majority of accidents are primarily or largely down to errors in flying/handling the problem is it any wonder that twins are little different to singles in the stats? I'm pretty sure that the minor safety advantage of the second engine is negated not only by the added complexity and variety of potential faults, but also the likelihood of mishandling them.
Two pilots, well, if they're trained as a crew maybe better, but two single-pilot pilots flying together because a charter client demanded it? Hmm...Sometimes a little knowledge is not so clever after all.
As to the "responsibility" of telling a client what he's flying in - why? Caveat Emptor. If he's concerned he'll ask. We'll be finding reasons to provide them type-related accident statistics next!