I don't think it's like this.the match day rule is that if the match is level on aggregate at Full time in the 2nd Leg, then if one team is leading on away goals, they have won. If not, then extra time is played. If at the end of extra time, the scored are level, then they look at away goals again. If one team hasn't won, then a penalty shootout takes place.
I think at extra time, previous games are reset.
let's say A vs B 1-0
B vs A 1-0
extra time 1-1
then goes to penalties
It's not the winner team A because of the away goal (1-2).
* edit
Not correct
@jcohen42 you 're right.