x1 is a grandparent of x2 which/who has another grandparent x3 such that x1 is related to x2 by relationship x4, and x1 is related to x3 by relationship x5 which is NOT their mutual mating/intimate partnering, under cultural/personal paradigm x6; x1 and x3 are mutual co-grandparents.
This would, for example, apply (for the x1 and x3 slots) to a maternal grandparent and a paternal grandparent of shared grandchild x2 in a simple family tree, but not to/as compared to a pair formed by two maternal grandparents, xor two paternal grandparents, of the same descendant. x1 and x3 are mutually symmetric if x4, x5, and x6 are appropriately reversed (if those are directed/not symmetric). See also: ".abvroi", ".abvele", "rorci".