Is it permissible for priests who normally celebrates the Extraordinary Form of the Mass to concelebrate Mass? Is it permissible for priests who normally celebrates the Extraordinary Form of the Mass to concelebrate Mass when saying the Mass according to the Tridentine Rite?? What about at some special occasions, such as the blessing of an abbot?
It seems fare to say that vast majority of priests who celebrate Mass according to the Tridentine Rite do not concelebrate their Masses with other priests.
However, I would like to know, if in fact it is permitted under certain circumstances, either in our actual times or even historically (perhaps in some ancient variations that were employed in historical Latin Usages like the Sarum Rite)?
Has Rome ever allowed this particular practice in the Extraordinary Form?

Latin liturgical rites
In the traditional (i.e. before Vatican II) rite, a mass of priestly ordination would be concelebrated by the ordaining bishop and the newly ordained priest(s). Specifically, they would pray the canon of the mass together. This is also the only case I've encountered where the canon is spoken loudly.
(There may be other traditional concelebrations, but I haven't seen any. The consecration of a bishop would be a possible example, but I have no knowledge about that.)