double shifts going up are not a problem on my bike - If I am accelerating pretty hard in 3rd gear I may just double toe it into 5th gear
if I am up to ~ 55 mph cruising speed. But, going back down is another story - these are not wet-clutch bikes, and they don't have modern synchronizers in the gearbox either. You will get some gear gnashing on any airhead transmission, I think, if you start going down to lower gears without taking a moment to match up engine and drivetrain speed at each gear. As Rob indicated, these bikes aren't designed to be shifted casually or overly fast.