Bp. Peter Elliot (in his Ceremonies of the Modern Roman Rite) places a second deacon at the priest’s side, which is in conformity with GIRM 172: “Carrying the Book of the Gospels slightly elevated, the Deacon precedes the Priest as he approaches the altar or else walks at the Priest’s side.” Since “The concelebrating Priests walk ahead of the principal celebrant,” (GIRM 210), this leads him to an order of
-deacon with Book of Gospels
-concelebrant(s)
-celebrant with deacon at his side
For pontifical (i.e., a bishop’s) Mass, however, the Ceremonial of Bishops prescribes at 129) an order of:
“-the deacon carrying the Book of the Gospels’
-deacons, if present, two by two;
-concelebrating presbyters, two by two;
-the bishop, walking alone …];
-a little behind the bishop, the two deacons assisting him”
So if we use that model we will place any further deacons who are *parati *(vested to fulfill their diaconal office at Mass as opposed to just being in choro) behind the Gospel book and before the concelebrants.
We may also note that while (name removed by moderator)'s assertion that a deacon of the altar would go immediately before the celebrant is rubrically incorrect (since a deacon without the Gospel book is said to walk at the priest’s side) it is practically correct. In my experience the near universal custom has been to place deacons ahead of the celebrant as opposed to beside him. I would say, however, that with concelebrants present as well it would be better, if the deacon will not be beside the celebrant, to place him before the concelebrants - the GIRM doesn’t speak directly to both “deacon without Gospel book” and concelebrants in the same procession, but if the deacon’s position isn’t clearly signifying his attendance on the priest by walking beside him (as the bishop’s two deacons who follow him) it strikes me as better to put him in hierarchical order where other deacons who are not attendants would go (ahead of concelebrants).
