Certainly multiple deacons are allowed, even encouraged.
From the Ceremonial of Bishops: “26 At a liturgical celebration presided over by the bishop there should be at least three deacons, one to proclaim the gospel reading and to minister at the altar, and two to assist the bishop. If more than three deacons are present, they should divide the ministries accordingly, [footnote 34 See GIRM, no. 71: DOL 208, no. 1461] and at least one of them should be charged with assisting the active participation of the faithful.”
(Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 24).
The footnote 34 above is referrring to the 1975 General Instruction of the Roman Missal, with different numbers to the 2002 GIRM. “DOL” means the book Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4 where this GIRM can be found.
Some references to multiple deacons in the 2002 GIRM which can be accessed from
romanrite.com/girm.html :
“109. If there are several persons present who are able to exercise the same ministry, nothing forbids their distributing among themselves and performing different parts of the same ministry or duty. For example, one deacon may be assigned to take the sung parts, another to serve at the altar; if there are several readings, it is well to distribute them among a number of lectors. The same applies for the other ministries. …”
“179 … If several deacons are present, one of them may place incense in the thurible for the consecration and incense the host and the chalice as they are shown to the people.”
“182. … when the distribution is completed, he immediately and reverently consumes at the altar all of the Blood of Christ that remains, assisted if necessary by other deacons and priests.”
“240. While the Agnus Dei is sung or said, the deacons or some of the concelebrants may help the principal celebrant break the hosts for Communion, both of the concelebrants and of the people.”