An alternate cause cannot be a weakener when the argument itself allows for alternate causes
Look at this example:
A study of marital relationships in which one partner's sleeping and waking cycles differ from those of other partner reveals that such couples share fewer activities with each other and have more violent arguments than do couples in a relationship in which both partners follow the same sleeping and waking patterns . Thus, mismatched sleeping and waking cycles can seriously jeopardize a marriage.
...
Look at this example:
A study of marital relationships in which one partner's sleeping and waking cycles differ from those of other partner reveals that such couples share fewer activities with each other and have more violent arguments than do couples in a relationship in which both partners follow the same sleeping and waking patterns . Thus, mismatched sleeping and waking cycles can seriously jeopardize a marriage.
...