I dont have 2 FXs, otherwise i would make a video of this, but heres my technique.
Its a basic move, but looks pretty cool.
What you need to do is a standard figure of 8 with your master hand (mine being the right), now when your master hand comes across your body for the first time of the figure of 8, you need to start the same line as you just took with your master, with your other hand. So they follow the same path through the air. And are at opposite ends of the cycle to eachother.
You could prbably do the same thing with yor sabers inverted, but you would have to be very careful!
If anyone wants to send me another FX i'l gladly make some videos! :P ;D :P (i wish)