Flightsafety and 411A.
I've been a corporate pilot for about 20 years and have always been crossed qualified on our aircraft.
As far as how many pilots to hire for two airplanes, the NBAA recommends 3 pilots per aircraft as an average. Our department, a major U.S.corporation, has 5 GV's and 2 F70's. We currently have 26 line pilots and 5 managers flying our fleet worldwide. All pilots are dual qualified and we have no problems handling 2 different aircraft types. Several pilots in our "country club" also fly in the U.S. Air Force Reserves, so they are actually flying 3 types.
What you need to look at is how many hours you plan on flying per year and what kind of trips (duty/flight hours) you plan on doing. With the GV's, we frequently add a third pilot on long flights (12+ hours) so our pilot/aircraft ratio is higher than the average.
As far as training costs go, it's usually cheaper. At FlightSafety, if you have a training contract, it allows you to pay for training on the most expensive aircraft in your fleet, and the cheaper aircraft is "free". I know this is true with FlightSafety International but this may not be true if you also use FlightSafety-Boeing. I don't know about insurance costs being any higher, I think they are only interested in how, and how much, you train.