I have read I-751 filing instructions and they are mentioning affidavits sworn to or affirmed by at least two people.
Are they necessary or just one of the supporting documents that may be submitted?
They are not "required".
You can obtain them, but they carry the least weight on the approval of your i-751. Id focus on financial documents, insurance docs, titles and deeds to property together, etc.
Do they have to be written by friends only? How about close relatives?
anybody who is either a USC or US LPR would be best.
I-751 Affidavits? Necessary or not?