The men, aged 52 and 21, were detained in Caernarfon on Friday following a joint operation by officers from the Metropolitan Police and North Wales Police.
They were taken to a police station in central London on Saturday and questioned about alleged offences under Section 4 of the Explosives Act.
Magistrates subsequently gave the police permission to hold them for a further 48 hours.
Officers are investigating the sending of two suspect packages, one of which was delivered to the Secret Intelligence Service's offices on Albert Embankment.
The second was intercepted at a sorting office in London and police have refused to comment on its intended destination, amid press speculation it was addressed to 10 Downing Street.
No details have been released about the two men who are being held, other than that they were arrested at separate addresses.
However, Wales on Sunday reports that they are a father and son of Asian origin.