A reference form has room for one signature. However, there's no reason that a letter of support could not be co-authored.
But 9 times out of 10, there is no need to co-author a letter b/c most PNMs are seeking only 1-3 letters of support and the alumnae know that will be sufficient. I can't imagine this happening on a very frequent basis. Even if the young lady has relatives high up in the organization, they know what will be acceptable and what will be overkill.
More recs =/= better chances
Being a quadruple legacy =/= better chances
Having good grades, activities and references = better chances
Someday, my friend's daughter will hypothetically go through recruitment. She has an ADPi grandmother and 2 ADPi aunts. I'd love to help her if she has any questions about recruitment, but I know that between being a legacy to Grandma and having 2 aunts in the sorority, she's covered.

I could probably be of more help digging other sorority alumnae references out of the woodwork for her. Or being around if she wants any non-family members' advice on recruitment. Or sending her a present.