What happens when stock is held by a shell company (or trust fund) and its property partially changes? What happens when one LTSE form buys up part of another L

What happens when stock is held by a shell company (or trust fund) and its property partially changes? What happens when one LTSE form buys up part of another LTSE firm that partially owned the first one to begin with? (I assume that rules are built with ADG in mind, general constructs are far more involved.)