Tues Sotah 8

The gemara gives the difference whether we Darshin the reason of the Pasuk is if the woman is naturally scared. We don't worry that she'll gain more confidence by her friend's confidence. However, if you don't hold of the Pasuk's reason, two Sotahs are forbidden in all cases.

Tosfos brings another difference. If the second woman is not a Sotah. She definitely not giving her confidence, however, the Pasuk implies that you cannot have a second woman along with her.

As we see the Gemara about taking out two Para Aduma, that the one that doesn't Darshin the Pasuk's reason forbids bringing two animals out together, even if the second one is a donkey (and not another cow.)