The two officials worked in Papaconstantinou's office when he was minister from 2009 to 2011.
The source said that the prosecutor Grigoris Peponis had summoned them "with great discretion" for questioning late on Thursday.
The investigation concerns the alleged falsification of a list of names of people with accounts at HSBC bank in Switzerland and who are suspected of having used Swiss accounts to evade tax.
Judicial information has indicated that the names of people close to Papaconstantinou had been removed from the list.
Revelations surrounding the matter on Friday led to the removal of Papaconstantinou, 51, from the membership of the socialist Pasok party.
On Monday, 71 members of parliament representing political parties on the right and the moderate left, and the Socialists, which support the governing alliance led by conservative Prime Minister Antonis Samaras, laid a proposal before parliament for the creation of a parliamentary commission of enquiry.
This would investigate the alleged role of Papaconstantinou in the matter.
If this commission is set up as a result of a vote to be held in the middle of January, it would look into whether or not the minister had been involved in the falsification of an official document and had not acted as he should have done.
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The controversy has badly shaken the world of Greek politics in recent days given that the International Monetary Fund and the European Union, which have provided bailout loans, have made fighting tax evasion a top priority in the effort to deal with the Greek debt crisis.
The list of names is known to Greeks as the "Lagarde list" after Christine Lagarde who was French finance minister at the time and now heads the IMF.
Lagarde had come into possession of the list of 2000 names which had been removed secretly from HSBC bank by an employee.