Turkish President Abdullah Gul announced the deal at a press conference on November 1 alongside Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari.
Gul described the development as "one of the most important results" of the summit and said that it was "an indication of how these two countries are working in sincerity."
Gul said the investigation will be coordinated by the two countries’ foreign ministries and will involve their intelligence services.
Kabul broke off talks with Islamabad in the wake of the killing, which it blamed on Taliban insurgents based in the Pakistani city of Quetta.
Karzai also accused the ISI, Pakistan’s intelligence service, of being involved in the assassination and of supporting the Taliban insurgency -- charges Islamabad has denied.
In welcoming the agreement to investigate the killing on November 1, the Afghan leader indicated that the murder had taken a major toll on already tense bilateral relations, but added that "the mechanism that we have [now] established for pursuing the matter of the assassination of President Rabbani...will lead us to a more fruitful and in-depth continuation of talks between Afghanistan and Pakistan."
When asked at the press conference to repeat his charge of Pakistani involvement in the killing, Karzai declined to comment.
In another step toward defusing tensions, the heads of the Afghan and Pakistani armed forces and intelligence services also held talks on November 1 alongside their Turkish counterparts.
A series of bilateral agreements in the fields of security and economics were also signed.
Zardari, in a jab at the United States and other Western nations that have urged Islamabad and Kabul to work together, said Turkey was better suited to facilitate progress between Pakistan and Afghanistan.
“There has to be a regional solution," he said. "The region has to take responsibility. Well-meaning friends, well-intended friends from afar, may want to help us [and] they want to support, but would not know our culture [and] they would not know our traditions."
"Turkey, being a friend [and] a fellow Muslim country in the brotherhood of Islam, I think is better placed to support and to guide wherever it is needed.”
Creating Political Momentum
While Zardari and Karzai expressed a willingness to continue talking, the Afghan leader indicated that talks with the Taliban in the wake of Rabbani's assassination would remain off the table for now.
“The person that came in the name of a messenger from the Taliban turned out to be a suicide bomber," he said before goin on to insist that there would be no further negotiations "until we have door that we can go [to] and knock on as the address for the Taliban and someone that we can recognize as the representative of the Taliban not a suicide bomber."
"Until that day, we have said that we will be talking our brothers in Pakistan to find a solution to the problems that we have,” he added.
On November 2, Afghanistan and Pakistan will participate in a regional foreign ministers summit on development and aid, again hosted by Turkey. Other countries to be represented include China, India, Iran, Russia, and the five Central Asian states of Turkmenistan, Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan.
Observers say creating political momentum in Afghan-Pakistani relations is crucial ahead of further international meetings on Afghanistan in Bonn, Germany, this December and in Chicago in 2012.
Ensuring that the situation keeps moving forward politically is also seen as vital as the United States and its allies prepare to withdraw most combat troops from Afghanistan by the end of 2014.