For a one sided NDA, which is typical between a contractor and larger client, there's no need for two signatures. You sign to agree that you won't disclose anything and then you obey the terms.
For a two sided NDA, which may be more typical for an equal partnership, I would recommend against disclosing anything before they have signed the agreement and returned a legal copy to you. Feel free to look at any information they give you, but provide no input or feedback until they sign. They're taking all the risks by not completing the agreement, but you should still assume you are bound to not disclose anything you see.
In general, I try to make sure that any contract I sign doesn't result in an unlimited binding agreement before I sign it. NDA's should have an expiration date or condition. Something like a SOW (Statement of Work) should have an expiration date on the terms if not signed and returned. And even then, it's a good idea for the termination clause in the SOW to give you an out if something changes between when you first signed and when it was counter signed.