It was generally for priests since concelebration was not allowed except at ordination Masses.
The Masses weren’t concelebrated in the strict sense of the word. One priest may have well been at the Canon and the other at the Prayers of the foot of the altar and another reading the Gospel.
Concelebration didn’t exist before Vatican II. They were for simultaneous celebration of Masses.
Before Vatican II, if there was more than one priest at Mass, wouldn’t the 2nd priest vest as a deacon?
This occurred at Regis in Denver. My brothers served the priest. I would go and would pick which priest I would receive communion from. One time I waited to long and the last priest left and I hadn’t received. My brother gave me trouble.
In defense, the picture was at a convention- the priests were in hotel rooms.
He could or he could celebrate his own Mass at another altar.
It did exist, it was just rather rare during the middle ages, used almost exclusively when newly ordained priests concelebrated their ordination Mass with the bishop. Previously to that, though, when concelebration was more common, it would have been done on the Eastern model, with all concelebrants assisting silently, not with them saying parts allowed or even stepping into the main role for a bit as our current rite allows.
If he did not celebrate his own Mass he would more likely assist as a server, since deacons were only used for solemn Mass.
Thanks.