@alaser the duplicate CP30
(a) for joint filers each is sent a notice for the FULL amount. ONLY the full amount ( i.e. not the aggregate / double amount).
(b) the CP30 should tell you where and how to pay -- the reason is the same as shown on CP30.
(c) you can always contact IRS at the "contact number " shown on the front of the notice. Whatever you do, Do NOT IGNORE the timing -- generally 30 days or whatever is shown on the notice.
Does this help ? Is there more I can do for you ?