Thanks con-pilot. I like the idea of 3 crews (6 pilots) for these 2 aircraft, with all pilots being dual rated. 3 crew members per aircraft would cover nicely things like sick leave, vacations, long trips requiring a relief pilot, being on-call (and not being on-call) for both the GV and the 757.
However, with 6 pilots rated on the 757, no one would get a great deal of time on it (240 hours per year / 3 crew = 80 hours per year per pilot). That might be an issue. You could have only 3 pilots (or perhaps 4) for the 757 and thus dual rated. It seems to me that might keep those pilots more current on the 757 because they'd fly more hours on it. But that could create another issue, there being 2 tiers of pilots. I'm not sure I like that idea.
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Safe flying to you...