The double vajra or vajra cross has its origins in Buddhist myth:
In the emptiness of space, a mild wind began blowing. Over time, it filled the great Void, growing in power over eons until it coalesced into the vajra cross. This weighty, immutable object is said to be the foundation of the universe or visvavajra (in Sanskrit, visva = universe). Depicted as an X or in vertical form (like a plus sign), it is also a symbol of protection.
The visvavajra is typically used as a seal or stamp. It is often inscribed or embedded at the base of Buddhist statues that represent protectiveness or are used to store prayers and relics within them.
The double vajra is commonly associated with the green Buddha known as Amogasiddhi (Enduring Accomplishment). His left hand lies in his lap; his right hand, palm facing outward before his chest, gestures in abhaya mudra, the sign of protection. Amogasiddhi’s consort is the Green Tara.