Are priests who committed a sin together able to validly administer the Sacrament of Reconciliation to each other?

Question

The Vatican recently released a report about the Holy See's knowledge and involvement in former cardinal Theodore McCarrick's rise to prominence despite allegations of sexual misconduct. The report quotes a letter from a psychiatrist who had been treating a priest who revealed an incident in which

...the young priest was shocked when he walked into the bedroom and found Bishop McCarrick engaging in sexual relations with another priest...My patient noted that the bishop and the other priest later administered the Sacrament of Reconciliation to each other.

_{Report on the Holy See's Institutional Knowledge and Decision-Making Related to Former Cardinal Theodore Edgar McCarrick (1930 to 2017), p. 121}

Obviously it is normal for priests and bishops to administer the Sacrament of Reconciliation to each other. However, is it valid for priests who committed a sin together to administer the Sacrament of Reconciliation to each other? Or must they seek the sacrament from a priest who was not involved in the sin?

It seems odd to "confess" a sin which the priest you are confessing to is obviously already aware of the sin since he committed the same sin. Moreover, in such a case it would be difficult/unlikely that the priest administering the sacrament would be able to assign an appropriate penance ("ah, just say a Hail Mary and you're good"), offer good advice to avoid the sin in the future, etc. Is there anything in canon law which forbids such a confession, or at least a recommendation to seek the sacrament from a priest who was not involved?

Answer 1

The case you describe is explicitly forbidden and declared invalid in canon law. (The sixth commandment is against adultery.)

can. 977 CIC The absolution of an accomplice in a sin against the sixth commandment of the Decalogue is invalid except in danger of death.

Additionally a violation of this rule is punished by one of the most severe sentence of the church.

can. 1378 §1 CIC A priest who acts against the prescript of can. 977 incurs a latae sententiae excommunication reserved to the Apostolic See.

So in the case described in the question both participants did not get the absolution, but incured (without a extra process) excommunication with all its severe consequences. (Something else may apply if there was an extenuating circumstance we do not know of, can. 1324 § 3 CIC. This may be relevant esp. for the victim of the sexual abuse.)

As this is explicitly regulated for sins against the sixth commandment and there is no similiar norm for other sins, the absolution of a participant in any other sin is valid. I fully agree that it is not a good idea to do that.

The norm dates back to the apostolic constitution Sacramentum Poenitentiae of Pope Benedict XIV. 1741. In this constitution he ruled comprehensivly about sexual relations in context of confession.

The rule was retained in the CIC 1917. When the current CIC 1983 was draftet, there were discussion whether to expand this to all mortal sins or at least to abortion. They leaved it as it was, having in mind the specific historic situation of Sacramentum Poenitentiae and the freedom of choosing a confessor for the faithful (see can. 991 CIC). Seemingly they saw only a necessity for such a rule in the case of sins against the sixth commandment.

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My source for the historic explanation is: Althaus, Commentary on can. 977 CIC. in: Klaus Lüdicke (Ed.): Münsterischer Kommentar zum Codex Iuris Canonici. unter besonderer Berücksichtigung der Rechtslage in Deutschland, Österreich und der Schweiz. Ludgerus Verlag. Essen, Germany (state: Juanuary 2008)

Answer 2

No, this is against 1983 CIC 977 (quoted in K-HB's answer).

1917 CIC 884 seems more general, saying that the absolution of an accomplice in unseemly sins is invalid:

The absolution of an accomplice in a sin of turpitude (peccato turpi) is invalid, except in danger of death; even in case of danger of death, outside of a case of necessity, it is illicit on the part of the confessor according to the norm of the apostolic constitutions, specifically the constitution of [Pope] Benedict XIV Sacramentum Poenitentiæ of 1 Jun. 1741.

^{Turpis means "ugly, unsightly, unseemly, repulsive, foul, filthy".}

However, Benedict XIV's constitution against solicitation in the confessional, Sacramentum Poenitentiæ, does say that the sin that cannot be validly absolved is a "peccatum turpe atque inhonestum contra sextum decalogi præceptum commissum" ("a shameful and dishonorable sin committed against the sixth precept of the Decalogue").