If you can roll your rs, by rolling your tongue as if to imitate the sound of a cat purring, you can double reed flutter tongue using this method.
Step One: Notice how the tip of your tongue rapidly vibrates back and forth behind your teeth on the roof of your mouth when you imitate the sound of a cat purring.
Step Two: Mouth open, repeat step one but with your top lip rolled over your top teeth.
Step Three: Repeat step two, with top and bottom lips rolled over teeth.
Step Four: Repeat step three with reed pressing against top lip only.
Step Five: Repeat step four with reed in instrument.
Step Six: Repeat step five and gradually pull lower lip up so less air escapes - and repeat a lot until no air escapes.
Step Seven: Repeat step 6 a lot, and after you can do that, start the note over again and again keeping mouth in same position.
Step Eight: Play a note and slur into a flutter tongue.
Step Nine: Play a flutter tongue at will.
Step Ten: Explore your register and dynamic range while flutter tonguing.
Be patient, good luck and enjoy liberating the Olympic spirit within.
P.S. (Special thanks to Sherry Sylar for teaching me how to flutter tongue. The above is an embellished/expanded version of what she taught me.)