The Communication Workers Union (CWU) said its members would walk out in the latest wave of industrial action linked to plans to franchise or close more than 70 Crown sites - the larger branches usually found on high streets.
A UK-wide stoppage will be held on August 24, staff in Scotland will strike on August 26, and union members in England, Wales and Northern Ireland will stop work on the 27th.
The five-month dispute is over closures, job cuts and pay, and the latest two full days of strike action will be the 10th and 11th rounds that staff will take.
Dave Ward, CWU deputy general secretary, said: "This is the first time we have announced two days of strikes at the same time and the first time we have announced back-to-back days of strike action.
"Coupled with the 90% Yes vote by members for industrial action short of strike, the message can't be much stronger to Post Office management.
"Crown post office workers do not agree with management's slash and burn approach and are prepared to take prolonged industrial action to defend jobs and services and win a fair pay rise.
"This is a company that made £94million profit last year, paid out £15.4m in bonuses to senior managers and gave its chief executive Paula Vennells a 37% increase in her earnings to £697,000, while frontline staff face job losses and pay freezes.
"It is a clear case of double standards and trampling those at the bottom for the benefit of those at the top."
Kevin Gilliland, network and sales director at the Post Office said: "We are extremely disappointed at the CWU's decision to call further strike action."