Only Agrias and Andro can benefit from this feat and only the character who has taken this feat is able to make use of the reaction this feat grants.
When either one of them casts a spell as an action on their turn, if the other Best Friend Magic Combo character is within 30' of the casting character, they can apply either the sculpt spell or the energy substitution on their ally as a reaction. Sculpt spells can only be applied by Agrias to Andro's spells, and Andro can only apply damage substitution to Agrias' spells.