When warming up for a test that requires a give and retake of the rein with both hands I start by giving only my inside hand and then only my outside hand before finally giving both hands together. By staggering up to the full give and retake your horse is prepared and more likely to not fall on its forehand when you give the rein.
This is also the method I use when first introducing give and retake of reins when schooling. Ideally eventually you can give and retake as you please without any change to the frame or way of going of the horse but whilst still unbalanced a step by step approach in my experience works best when needing to prepare to perform it in a test.

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