IIRC, the only examples of Elu Ve'elu (אלו ואלו דברי אלקים חיים) appear in the Talmud in the name of the Heaven (see here):
Machlokes B"H and B"S (see for example Eruvin 13b): ...a Divine Voice emerged and proclaimed: Both these and those are the words of the living God.
Pilegesh Bagiv'ah (see Gittin 6b): G-d (or Eliyahoo) testified that R' Evyatar and R' Yonatan are both "Elu Ve'elu".
As we see in those examples the saying is very specific and focused on an argument between two Rabbis and does not extrapolate on all Rabbis and all Rabbinical arguments.
I'd like to know, who extrapolated/applied the saying on all Rabbis and all Machlokos, that in all arguments both sides are right?
(Thanks DoubleAa for accentuating this point).