According to Wikipedia:
"A bilateral NDA (Sometimes referred to as a mutual NDA or a two-way NDA) Involves two parties where both parties anticipate disclosing information to one another that each intends to protect from further disclosure. This type of NDA is common when businesses are considering some kind of joint venture or merger.
When presented with a unilateral NDA, some parties may insist upon a bilateral NDA, even though they anticipate that only one of the parties will disclose information under the NDA. This approach is intended to incentivize the drafter to make the provisions in the NDA more "fair and balanced" by introducing the possibility that a receiving party could later become a disclosing party or vice versa, which is not an entirely uncommon occurrence."