Not entirely sure what you're getting at here. It almost sounds like you seem to think that Adult Link existed in the Adult Timeline following him being sent to the child timeline....
Once Ganon was defeated, Link can't have interacted with the Master Sword again. In the Adult timeline he didn't exist anymore (which is what caused me to question whether the original master did as well) and in the Child Timeline he left it in the PoT where it was sealed away.
I'm saying Link could have chosen to remain an adult and remain in Hyrule till his death. Or he could return to his childhood by pulling the Master Sword from its pedestal and journeying to Termina for the Child Timeline.
OK. So if there can only ever be one, how is there one in both the Child and Adult timeline?
This comment doesn't really do anything to disprove the theory. Rather it works quite in it's favour, as if it was true there was a time in the Adult timeline where there was no Master Sword at all and the Child Timeline housed the original. Thus, a new one had to be forged in order to seal Ganon's magic.
Sorry to say, but you're misunderstanding something. The Child and Adult Timelines are split where Link either leaves the Master Sword sheathed in its pedestal (Child) or takes it with him (basically for the rest of his life; Adult). Now these are two separate possible realities Ocarina of Time Link could have taken:
1) Leave Hyrule as a youth still and travel to Termina, which then moves on to the events of MM and TP in the Child Timeline. But he leaves the MS in the Temple of Time, until it reappears in Twilight Princess.
2) Stays in Hyrule and most likely lives out his days there until his death in the Adult Timeline. After he dies the Master Sword is taken and hidden beneath Hyrule Castle because Ganon's seal was broken, and Hyrule was being flooded, which then moves on to the events of WW, PH, and ST.
The placement of the other games is undecided to me, but that, so far as we know, still leaves one Master Sword.