The issue is more one of what an attorney is ethically authorized to disclose pursuant to professional ethics rules (Rule of Professional Conduct 1.6 as enacted locally) more than one of attorney-client privilege. Attorney-client privilege governs what someone can or cannot be compelled to disclose without their consent (usually by subpoena). Attorney confidentiality, in contrast, governs what an attorney can disclosed when not compelled to do so by something like a subpoena.
Generally, disclosure of confidential client information is allowed if it is in furtherance of the representation or authorized by the client (there are also other exceptions but those aren't really any different in a two lawyer v. one lawyer situation). Often when two attorneys represent the same person (and the disclosure won't waive the privileged nature of the information vis-a-vis third-parties under a joint defense theory), disclosures from one attorney for a person to another attorney for a person will do that.
In practice, however, the analysis could be quite fact specific and there isn't really a bright line rule governing when it would or would not be allowed.