Was adelphopoiesis a plural rite? Was the rite of "brother-making" exclusive to two individuals, or could a man make more than one other man his brother? Is there any part of the rite that requires you to be nonunited previously?

Adelphopoiesis, or Ordo ad Fratres Faciendum as it may have been known in the Western Church, was used to unite two men as brothers in spirit. Some sources that I have read have described this rite as a method of establishing familial bonds where they did not previously exist, for the legal purpose of extending an inheritance to a close friend for example, much like adoption, but an adoption of a sibling instead of a child.

The customs apparently differed in different areas and locations. But generally the rite existed wherever sworn kinship was practised, and may have also been practiced as a pact between soldiers or "brothers-of-arms". 

Modern interpretations have tried to illustrate the rite as a union of homosexual couples, but there are cases where individuals were both heterosexually married and united to a brother. There are also many arguments that distinguish the rite from marriage, even though the ceremony has many aspects that are similar to a marriage ceremony. 

My question is: Can you have more than just one brother by this rite?

Related: What is the case in favour of adelphopoiesis as a relationship akin to marriage?
Yes adelphopoiesis could be used to join multiple persons as siblings. It was also used for children who were ether orphaned or needed to be sent abroad due to war or economic calamtities (or whatever). They would have a blessing to make them siblings and have each others backs.

The most famous case is that of Photius Kasavales Fisk, who was a U.S. Navy chaplain in the 1870's. At 15 years old he was orphaned from the Greco-Turkish War of 1821, and a Protestant missionary (Pliny Fisk) found him living amongst relatives in Malta. He secured Photius an education in America, where the local Greek priest, after examination of Pliny Fisk's intent, also sent his 11 year old son Anastasius together with Photius for a better life. It was agreed that the boys would not be separated upon arrival in America, and the priest performed the rite uniting the boys as brothers. 

You can read about it from Photius Fisk biography, written in 1891. Note this blessing has nothing to do with marriage or sexual unions (that's an invention of the last few decades). It is more akin to an adoption ceremony.  Read Photius Fisk: A Biography, by Lyman F. Hodge pages 23-26.