in final fantasy 7 a one professor Hojo theorised that
the cells of JENOVA if ever detached from the main body
would seek to re-assemble" and anyone who was implanted with JENOVA cells after sephiroths "death" (being controled by the JENOVA cells) would travel to the
northern cave were JENOVA once was to reunite the lost parts. The sephiroth clones who had the cells form Hojo's expiraments traveld to the northern crater and were killed my the manifestation of sephiroth and dumped into the life stream so basicly the reunion theroy is: once any cells of JENOVA are removed, they would seek to re-attach, thus causing the "reunion"
p.s. the "sephiroth clones" are the guys in the black robes.