complex vertical group structure ( from video lecture)

I have a doubt in the video it is said that if the SS acqn is before the S acqn , that previous date can be ignored… effectively both date becomes same … so in W3 of SS we take fair value of consideration of SS as a % of parents share in Sub… so if the sub already controls the SS before the acqn by the parent… how will we treat the SS fv of consideration…?? can anyone help…