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 mating/intimate partnering relationship x5, under cultural/personal paradigm(s) x6; x1 and x3 are mutually partnered co-grandparents.
This would, for example, pertain (in the x1 and x3 slots) to two maternal grandparents of x2, whether they are biologically so or just by marital law or some adoption, etc., but not to/as compared to a pair formed by a maternal grandparent and a paternal grandparent in a simple family tree. See also: ".abvorci", ".abvele", "rorci".