Are the man and his father considered unmarriageable (mahram) to a woman who was nursed by the man's mother, knowing that he (the man) was also nursed by her (the woman's mother), and the number of nursing times is unknown?

Prohibition is established by five full, satisfying sucklings. Doubt concerning the number of sucklings does not establish prohibition, because the default is the absence of suckling. So, if a man and a woman doubt whether the suckling from the other's mother reached five sucklings, then propinquity (mahramiyya) is not established between them.