I have flown passenger ops in 135 single pilot in Cessna twins and King Airs. I feel it is safe. You have to be on top of your game in weather and or busy airspace. I took the time to organize the charts I would need in order, and put them where I could get to them easy. The empty co-pilot seat became a desk.

At the request of the customer, we would have a SIC, but we still trained and checked for single pilot ops. We would work out our procedures for CRM on a personal level. Due to the small size of the company, all the pilots knew each other.

In the King Air 200 that I flew then, and the one I fly now, under 135 the insurance requires two pilots.

While I think single pilot can be safe, I feel two pilot operations are safer if CRM is put into practice. Having help with comms, charts, navigation, checklists, and another set of eyes to watch outside and watch you are a good deal.