If the applicant simultaneously appoints two patent practitioners, applicant should indicate with whom correspondence is to be conducted by specifying a correspondence address. See MPEP §§ 403, 403.01(a), and MPEP § 403.01(b).
If, after one patent practitioner is appointed, a second patent practitioner is later added by submission of a new power of attorney appointing both practitioners, correspondence will be mailed to the latest correspondence address of record. See 37 CFR 1.33 and 1.76. Note that if the later-filed power of attorney only lists the second practitioner, the later-filed power of attorney serves as a revocation of the earlier-filed power of attorney, even without an express revocation of the power of the first patent practitioner.