Tosfos on daf 2b in dibur hamaschil atihave bei trei asks why cant you just make any two guys witnesses for the signatures. Tosafos answers that it is related to the concept of shlichus. How does that answer the question because it shouldnt matter if they were messengers or not as long as they can validate the get.
TOSFOS explains in the name of the RIVA that the two people who bring the Get do not have to testify at all that it was written and signed in front of them. Rather, they are believed to say that the husband sent them with it (as Shelichim to bring the Get), and thus we assume that it was written and signed properly. They are believed to say that the husband sent them with the Get because of a Migu that they could have said that he divorced her in front of them (for which they would be believed).
M. Kornfeld