I came relatively late to multiple tonguing technique. As a young pup I had a pretty fast single tongue that got me through most situations.
It wasn’t until my late twenties actually that I seriously began to practice double tonguing. The fast single tongue of my teens was slowing down and I needed to learn to double tongue. For weeks on end I worked on my TEEs and KAYs, in all ranges of the instrument.
A very useful technique was playing scales using nothing but the KAY articulation. By happenstance it was this concentration that lead to a unique discovery. Lateral tongue movements in addition to the usual back-and-forth motion with double tonguing can increase the velocity and rapidity of the technique.
Front… back… right-side… left-side…
My articulation speed increased nearly two-fold! I was like a freak of nature for several years; orchestral pieces like Scheherzade and Alborada del Grazioso were no longer a challenge.
Alas, without concentrated study this amazing ability is lost quickly. Over the years my verisimilitude in this technique has both waxed and waned.
I did manage to make a primitive video of it once and upload it to YouTube. If you would like more information on this lateral tongue technique, please see this video.