Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon XIV

Canon XIV.

If a bishop shall be tried on any accusations, and it should then happen that the bishops of the province disagree concerning him, some pronouncing the accused innocent, and others guilty; for the settlement of all disputes, the holy Synod decrees that the metropolitan call on some others belonging to the neighbouring province, who shall add their judgment and resolve the dispute, and thus, with those of the province, confirm what is determined.

Notes.

Ancient Epitome of Canon XIV.

If the bishops of the province disagree among themselves as to an accused bishop, that the controversy may be certainly settled, let other neighbouring bishops be called in.

Zonaras.

When any bishop shall have been condemned with unanimous consent by all the bishops of the province, the condemnation cannot be called into doubt, as this synod has set forth in its fourth canon.  But if all the bishops are not of the same mind, but some contend that he should be condemned and others the contrary, then other bishops may be called in by the metropolitan from the neighbouring provinces, and when their votes are added to one or other of the parties among the bishops, then controversy should be brought to a close.  This also is the law of the Synod of Sardica, canons iii. and v.

Aristenus.

Every bishop accused of crimes should be judged by his own synod, but if the bishops of the province differ, some saying that he is innocent and some that he is guilty, the metropolitan can call other bishops from a neighbouring province that they may solve the controversy agitated by the bishops.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars II., Causa vi., Quæst. iv., can. j.  The Roman Correctors note that the Latin translation implies that the neighbouring metropolitan is to be invited and say, “But, in truth, it hardly seems fitting that one metropolitan should come at the call of another, and that there should be two metropolitans in one synod.”