Building on a peace accord already signed with southern rebels, the power-sharing deal struck by Khartoum and the SPLM rebel group to end 21 years of civil war allocates a proportion of seats in a new national government to other parties.
Sunday's agreement with the opposition National Democratic Alliance (NDA) will pave the way for discussions on how to divide up those seats and how to integrate opposition armed forces into the national army.
NDA spokesman Hatim al-Sirr said the two sides would set up joint committees to deal with both issues.
"On the military forces, it was agreed to form a joint committee between the government and the NDA to reconcile the points of view," he said.
Partners for peace
The committee on political representation will discuss the proportion of posts the NDA will hold in the legislative and executive bodies during a transitional period, he added.
Apart from the SPLM, the other main group in the NDA is the Democratic Unionist Party, one of the big traditional parties in the Arab north.
Other members include the Sudanese Communist Party, the Baath Party, the Beja Congress from the east of Sudan, the Sudan Liberation Movement (SLM) from Darfur in the west and an alliance of southern parties independent of the SPLM.
Sudan's other big northern party, the Umma Party, is not a member.
The proportion of SPLM seats has already been fixed in the power-sharing deal signed last week as have arrangements for integrating former SPLM rebel fighters into one of two armies.
The NDA was a serious challenge to Khartoum in the 1990s, when it launched a military campaign into the east from Eritrea. Al-Sirr said it still had thousands of fighters under arms.
The agreement enshrines basic principles of governance such as pluralist democracy, the equality of citizens and the peaceful transfer of power from government to government.
Talks between the NDA and the Sudanese government have taken place in Cairo on and off for months, in parallel with separate negotiations in Kenya with the southern SPLM and in Abuja with two Darfur rebel groups, including the SLM.
Al-Sirr added the agreement was signed by NDA deputy chairman Abd al-Rahman Said and Sudanese Minister of Federal Relations Nafi Ali Nafi. A complete agreement will be signed at a higher level on 12 February.