When two thunderbolts (Vajra meaning thunderbolt) are crossed it is called Viswa Vajra. It is the emblem of Amoghasiddhi.
Amoghasiddhi is the fifth Dhyani Buddha in order. He represents the cosmic element of Samskar (Conformation). He always faces north and is the embodiment of the rainy season.
In Buddhist Tantra the word generally designates Sunya or Void which cannot be cut, cannot be destroyed but which destroys all evil.