The two officials are Syria's former head of intelligence in Lebanon, Rustum Ghazala, and his deputy, retired colonel Samih Kashaami, both of whom were heard by the commission in December.
Safwan Ghanim, Syria's ambassador to Austria, speaking to AFP said: "I think it has begun," adding he did not expect the hearings to last beyond Monday.
The December hearings involved three other Syrians: Abd al-Karim Abbas, head of intelligence in Palestine, Zahir Yusuff, head of communications and Jamih Jamih, another of Ghazala's deputies.
The commission, led since last week by Belgian prosecutor Serge Brammertz, also wants to speak to Syrian President Bashar al-Assad and Foreign Minister Farouk Al-Shareh.
On Saturday, a senior US diplomat warned that Syria risked further UN Security Council action unless it stepped up co-operation with the investigation.
Two preliminary reports by the commission's first chairman, Germany's Detlev Mehlis, implicated Syrian and Lebanese intelligence officers and cast doubt on Damascus' co-operation with the UN inquiry.
Last week, a Syrian, Ibrahim Michel Jarjura, was arrested for giving false evidence to the enquiry, bringing to 12 the number of people detained in connection with al-Hariri's assassination in Beirut in a car bomb on 14 February 2005.
Among those arrested are four Lebanese generals who were main figures in Syria's security system in Lebanon.