The second device was found at a depot in Lisburn a day after a bomb was discovered in an envelope in a sorting office in Londonderry.
It is understood both were addressed to Maghaberry high security prison in Co Antrim. The finger of blame will again be directed at dissident republicans opposed to the peace process.
Stormont's First Minister Peter Robinson and Deputy First Minister Martin McGuinness said: "The people behind these letter bombs are opposed to the democratic will of the people and want to drag us back to the past."
The alert at the Linenhall Street facility in Lisburn came after a letter bomb was made safe at the depot in Londonderry.