As I understand it...
Regardless if you 1 or 2 up the amount of twist in the main should be adjustable according to your boat speeds and conditions and your jib should copy that same setting to aviod bad air flow onto the lift aera behind your main. A Sail that can change shape from flat to full is the best cut main but it needs a fully adjustable jib to best compliment watever shape the main sail is set to.
A Flat cut main is easier to change too a fuller main with softened batterns and less tension. Where as a Full main requires Very Stiff battens and lots of tension but you will struggle to flaten the sail completly and will procced to stretch and wear out you sail.