The deal comes amid fears that the bloodshed in Sudan's western Darfur region is increasingly spilling over the border into Chad.
Chad says Janjawid fighters, operating in Sudan's Darfur region, regularly mount raids into Chad, attacking civilians.
Sudanese-backed Chadian fighters also stage attacks into Chad, while Chad has mounted army assaults into Darfur.
The Sudanese government denies supporting the Janjawid, but the International Criminal Court in The Hague has accused a member of the Sudanese security forces of being a Janjawid commander and a junior Sudanese cabinet minister of organising financial backing for the group.
Thursday's deal in Saudi Arabia is aimed at committing Sudan and Chad to support the African Union's efforts to restore "stability in Darfur and the border areas".