For example, Strength in Certainty on the defense and Shiba Aikune xp3 both target two cards at the same time. What happens if those are the only two cards at the battlefield that get targeted; is it possible for a Seat of Power or Shosuro Aroru xp to redirect the action so that one card is targeted twice?
redirecting multiple targets
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
For example, Strength in Certainty on the defense and Shiba Aikune xp3 both target two cards at the same time. What happens if those are the only two cards at the battlefield that get targeted; is it possible for a Seat of Power or Shosuro Aroru xp to redirect the action so that one card is targeted twice?
No. That was never a legal choice to begin with, so it isn't a legal configuration to redirect to.