Innocent III (Pope 1198-1216) made his views on concelebration clear enough; so did S Thomas (see earlier posts). But the former, it could be argued (Durandus did), was writing as a private theologian; and as for the latter, despite his eminence, Cajetan disagreed with him.
Benedict XIV (Pope 1740-1758), undoubtedly one of the half-dozen most erudite men ever to grace the Cathedra Petri, left nothing to chance. As well as in his monumental work de Sacrosancto Missae Sacrificio (Liber III caput xvi), he made his teaching about Concelebration very clear in two magisterial documents, the encyclicals Demandatam (12 December 1743; paragraphs 9-10) and Allatae (26 July 1755; paragraph 38).
The basis of the Sovereign Pontiff's teaching is his conviction that the Eastern and Western churches are at one in this matter so that the practice of the Byzantine East can throw definitive light on the significance of our Latin practice: "It was once a rite common to the Western and Eastern Church equally, that presbyters should offer the Sacrifice of the Mass together with the bishop [copious references follow] ... at the present moment the Rite of Concelebration has grown obsolete in the Western Church, except in the Ordination of Priests which the bishop performs, and in the Consecration of Bishops, which is carried out by a bishop with two other bishops assisting".
He points out that the obsolescence of Concelebration in other circumstances in the West is comparatively recent (temporibus haud ita ab aetate nostra remotis), and that previously the 'disciplina Ecclesiae Occidentalis' demanded (postulabat) that on major solemnities, when a bishop was celebrating, presbyters should celebrate together with (una cum) the bishop - and the words of Innocent III are one of a number of exempla that he draws in to support the assertion. Not that he believes Concelebration is necessarily confined to Concelebration cum episcopo. He had to deal with a request from Byzantines who desired to celebrate the Eucharist daily but who lacked enough altars to do so (Byzantine custom insists that every Eucharist be celebrated on a 'fasting' altar). He categorically refuses them permission to celebrate twice on the same altar and advises them instead to concelebrate "with a bishop or with another priest".
He insists that concelebrants should vest as celebrants and utter the words of Consecration "just as if they were saying Mass on their own [perinde ac si sacrosanctum sacrificium singulatim conficerent]". Benedict explicitly condemns the wriggle-argument that such priests are merely saying the Words of Consecration "materialiter et recitative", insisting that they utter them "significative". They are true celebrants, albeit secondary ones [etsi secundarii, tamen vere celebrantes].
It is a laudable desire to shape ones liturgical praxis by the authentic customs of the Roman Liturgy. But this does not require that a priest should decline to concelebrate the Maundy Thursday Masses with his Bishop and Presbyterium.