We’re into some pretty fine points here. Exercising some theological restraint, I’d say that to my knowledge there doesn’t seem to be anything objectionable with the idea of offering the indulgence granted by the Church for the dead, while offering the satisfaction gained by the underlying act to God on behalf of another living person.
Here are a couple of other scenarios that seem to indicate that your scenario above is possible:
- If one were to pursue an indulgence for dead, but without the idea of asking God to apply the satisfaction of the underlying act to someone else, it would seem that by default the satisfaction would benefit you, yourself. Thus we see that two different people can seem to receive the benefit of one act.
2)If one were to perform an act with the intention to ask God to apply the satisfaction thereby gained to another living person, but at the same time having the required habitual intention of acquiring and indulgence (a minimum intention) and one manages to fulfill the conditions of an indulgence attached to the act, and thus actually gain the indulgence for oneself (unknown to you!) then here again we have one act which benefits two different people.
There is a fine point, however, that we should keep in mind. It is one thing to speak of the satisfaction for sin gained and another thing to speak of merit gained. Although they are closely related to each other in an act, they are distinct. When we speak of merit strictly we really mean merit de condigno. That seems to be only won for oneself – meriting an increase in sanctifying grace, an increase in heavenly glory, and eternal life (if dying in grace). Whether or not one can “merit” (i.e. *de congruo) *something for another seems to be an open question. Much more certain is that we can pray for another.
FAIR WARNING: The above is mostly speculation on my part. Take it with a grain of (blessed?) salt.
Please, I welcome additions, corrections, clarifications, reprimands, etc. from my fellow posters.