in the מחלוקת of כל שבעה קורע or כל שלושים קורע the Gemara seems to say that the reason one can just add onto an existing ripping is because it is getting sewed up to some extent. meaning that adding onto an existing קריעה doesn't fulfil the din of קריעה only that since it was fixed either by being נשלל or מתאחה and therefor now a full קירעה can be performed.
the problem is that why then did the gemara say in the beginning of the amud that if one ripped for a father and a son the part that was for the father cannot be מתאחה but the part for the son can be fixed. for example, in the case of the intial rip made for the son and the extension of that קריעה for the father why is the initial rip for the son fixable since this same rip is now part of the 2nd קריעה for the father.
it seems that the second קריעה isn't actually using the 1st as part of it, rather only the additional rip is used to fulfil the din of קריעה its just that during שבעה or שלושים this rip is "in use" for another מת and therefor the extension of this same rip cannot be used for another מת. (this all is the meaning of a comment in the "הערות" - in my eyes) still this extension only has an importance of a rip since this garment already has a rip of a tefach והבן.
another wonder is both of the answers of the gemara in the latter part of the amud: ...התם משום כבוד אשה and התם משום כבוד אביו ואמו forgive me for not quoting the שקלא וטריא but the problem of the gemara was that by the case of the woman even during shiva she should be able to rip since she already fixed it and conversely by the father or mother one should not be able to rip even after 30 days since it isn't fixable - the question is understandable but what is the answer?
again it seems that this extra din of honor to a woman and conversely to father doesn't change the status of the actual קריעה and therefor a woman may not rip even during shiva for her rip is still "in use" for the 1st מת and conversely the rip for a father and mother is no longer in use after שלושים even if the din of honor doesn't let him fix it.