Sorry to just jump in here, but I have always flown more than one type of aircraft in every job I have had for the last 20 years.
Corporate flight department from 1977 to 1986: Company owned a Westwind 2, Jet Commander, DC-3 and a Bell 222. I was the chief pilot and was current on the Westwind, Jet Commander and the DC-3. The Bell 222 pilot was current on the 222 and the Westwind.
United States Marshal Service 1988 to 1998: The USMS operates B-727s and Sabreliner 80s. I was a line captain on the B-727 and the Sabreliner (75% on the 72). We attended recurrency training on both airplanes every year.
Corporate flying 1998 to current: We operate a Sabreliner 65 and will soon start operating a New Falcon 900EX. All pilots will be current in both aircraft.
I really feel based on my experience that pilots can stay current and safe on two different types of airplanes. I also agree that 3 crew members per aircraft is best.