I read the cards a little differently. This is all based on my own views, though.. not actual hard rules.
Shii-Cho Training over rides the normal requirement of only targetting one unit.
Targeted Strike over rides the normal requirement of participating units.
My flow of logical would be as follows..
Luke Skywalker is focused to strike. He has several unit damage icons to apply. I read Shii-Cho Training which states I can go ahead and target multiple targets instead of one. Great! That over rides my normal limitation of targeting only one unit. I read Targeted Strike, which states that I can apply damage to a unit not participating in the combat. The card uses "a" not "one".. so I don't believe this is a numerical hard limit. Its verbage is just referring to the normal requirements of applying unit damage to a single target, which already no longer applies to Luke due to Shii-Cho training. So I would resolve the unit damage as being spreadable thanks to Shii-Cho, and targetable on non-participating units thanks to Targeted Strike.
Targeted Strike I believe not only modifies the normal rulebook, but also the rules on Shii-Cho Training as they are just enforcing the normal rules for targeting, not creating a new hard limit itself. And at the same time, Shii-Cho Training is modifying both the normal rules outlined in the rulebook along with the reference to those rules on Targeted Strike. For the same reasons. I don't think Targeted Strike is creating new limitations, just references the normal rule of targeting just one unit.