Having crews X-qualified can be both good and bad.
I flew at a time A CE525, B737 and some turbo-props for a year or so. I did not find it diffucult to keep up to speed. Requires a lot of interest from pilot to keep reviewing abnormal procedures. I donīt think that operating a B757 and G IV or V with crews that are qualified on both airplaes would be a problem.
Safety is an important issue, but with proper initial training and continuous recurrency training this would surely not be a problem. A proficiency check in each type every year. That adds up to a procheck every 6 months for any given crewmember. Now the question would be if it was cheaper to have separate crews for both types as training is not cheap.
I know for a fact that they send their crews for type specific recurrency training once every 2 years and have them obtain a type rating every year in another aircraft of the same category and class. What I remeber is that heavy iron bizjet types and recurrency is horenndously expencive. GIV @ over 30000 USD initial and 12000-15000 USD for recurrency. When a B747 type can be obtained for 7900 USD makes a big difference.
JJ