This spectacular manouvre was invented by the Safety Acro Team in 2000. Two pilots start to make Deep Spirals in front of each other, around the same turning center. As the spiral accelerate, the gliders approach, and it could be so strong that they may touch each other.
It’s a very dangerous manouvre. If the sensible according between the two pilots brake up, they can easily crash.
The extended version of the Synchro Spiral is the Synchro Ground Spiral, wherein the pilots continue spiraling down to the ground, and they end up in a Ground Spiral.