If i wasnt lied to it prioritizes the one with highest combat readiness, sadly dont have him so cant test it but try to play around with teams that have 1-2-3 step in speed and see if the one with highest combat readiness is the one to get it

S2 description: Grants increased Attack and Critical Hit Chance to the caster and an ally for 2 turns, and increases Combat Readiness by 30%..

It didn't even say to the Highest atk ally... Mean that it's random.

tywin S2 targets the next non-soul weaver unit. it doesnt care who has the highest attack.