Normally, I would favor offering only one QB per recruiting class (unless the intention was to move one to another position). However, with Barnett dealing with an injury his senior season (apparently a hip problem), I could see offering a second QB.
I have talked to two high school coaches in SD, (whose opinions I trust), who say that Sumner is as good a high school passer as they have seen. Both are former college FB players. Whether Sumner fits Coach Herman's criteria, I wouldn't know. I do trust that Austin Sumner is getting a good evaluation by staff.