They were held as part of a planned police operation in South Down, at Kilcoo near Castlewellan.
Nobody has been charged with the raid on the bank's HQ just before Christmas, which was blamed on the IRA by Chief Constable Hugh Orde.
But these arrests are believed to be considered a significant line of inquiry in the police investigation.
Kilcoo, where the two men were detained last night, is close to the area where a bank official's wife was held hostage as part of the operation to clear out the bank's cash vaults in the centre of Belfast.
Cash seized in Co Cork last February was linked to the raid, but virtually all of the missing millions is unrecovered.
Even though senior IRA men in Belfast were suspected of heavy involvement in the raid, detectives do not have enough evidence to charge them.
But according to security sources in Belfast, there is a firm belief that the inquiry team could, at some stage, bring to trial at least some members of the 30-man gang who played a peripheral role.
Last March the Northern was forced to replace all its £10, £20, £50 and £100 notes with new notes carrying a different logo.
The Provisionals have always denied any involvement in the raid.
Even though the organisation has disarmed and officially ended its campaign of terror, security chiefs insisted the move by the IRA to call a halt would not influence any decision to charge anyone in connection with the robbery, provided they had sufficient evidence.
About £60,000 in used Northern Bank notes recovered in Co Cork last February as part of an investigation into IRA money-laundering has been linked to December's raid in Belfast.Reuse content