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.
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?