I still think the easiest solution here is simply a matched pair of Performances. For starters, you aren't limited to only one Variation for your double time, so you have a bit more flexibility and options, and you don't have to chop up anything...
Not sure about Yamaha's, but my Roland goes seamlessly from one Performance to the next and back, no glitches, no nothing. You can set up which Variation of the double time style will cue up when you switch Performances, so no hunting and pecking there.
At least on Roland's, I'd be willing to say this is MUCH faster to set up than copying Parts from one style to another. And Roland style editing seems to be able to do the same thing as the Yamaha (mix and match).
But, whatever floats your boat...
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An arranger is just a tool. What matters is what you build with it..!