Their previous relationship was dominated by civil war and violence. But the fall of Sudan's long-term ruler Omar al-Bashir is changing the country's relationship with South Sudan - to the benefit of both nations.
When Sudan's new prime minister, Abdalla Hamdok, arrived in the capital of neighboring South Sudan in September, he expressed how "very delighted" he was to be in his "second home, Juba."
In the past, delight probably wasn't the primary feeling of Sudanese politicians towards South Sudan.
The two countries have had a particularly acrimonious relationship - South Sudan split from Sudan in 2011 following decades of brutal civil war fought between Sudan's government in the predominantly Muslim, Arabic-speaking north and primarily Christian rebels in the south.
Now, their relationship is visibly thawing.
"We are looking for a very strategic, very distinguished relationship between our two nations and the sky is the limit for this relationship," Prime Minister Hamdok said during his visit.
Hamdok heads an 18-member transitional government formed after the overthrow of Omar al-Bashir, who ruled Sudan from 1989 to April 2019.
Bashir's removal signals a chance for the two neighbors to mend their relationship and also work together to solve the internal conflicts raging in each country - which have at times been supported by each other's governments.
"Both countries are actually dependent on a good relationship with each other," said Marina Peter, Sudanese expert at the German relief organization, Bread for the World.
Sudan backed rebels in South Sudan
In 2013, just two years after gaining independence from Sudan, South Sudan was plunged into a civil war pitting soldiers loyal to President Salva Kiir against fighters who support former vice president, Riek Machar.
It's estimated that some 400,000 people have been killed in the violence and more than a third of the country's 12 million people have fled their homes.
Under the governance of Bashir, Sudan backed Machar's rebel group as well as other elements opposing the South Sudanese government.
"For Kiir, it's important that Machar no longer receives support from Sudan, especially in the form of weapons," Marina Peter told DW.
South Sudan supported armed groups in Sudan
At the same time, rebel groupings inside Sudan also received aid from South Sudan.
In particular, the Nuba Mountains area, located in Sudan's far south, has historically and culturally had strong ties to what is now South Sudan, who supported the Nuba rebels in their fight against Bashir's forces.
The improved relations between Sudan and South Sudan are already evident, says former EU Special Representative for Sudan, Rosalind Marsden.
"Both countries are acting as mediators to help resolve each other's conflicts - not least because each government has influence over opposition forces in the other country," Marsden told DW, adding that this was a "golden opportunity" to see an end to the conflict in Sudan.
Since October, South Sudan's President Kiir has been hosting and brokering negotiations between Sudan's transitional government and an alliance of Sudanese armed groups.
The talks, which are expected to resume on November 21, have led to agreements to cease hostilities and open up humanitarian corridors in Darfur, Blue Nile and the Nuba Mountains.
Sudan mediating between South Sudanese parties
Under Bashir, Sudan's government, along with Uganda, had already started mediating between Kiir and Machar who signed a peace agreement in September 2018 and agreed to form a transitional government of national unity.
More recently, the head of Sudan's Sovereign Council attended face-to-face talks between Kiir and Machar hosted by Uganda to promote the formation of a South Sudanese unity government.
(This new government should have been formed by November 12, 2019, but has now been pushed back by 100 days.)
"It's clear the new government in Khartoum is stepping up to that responsibility and making a positive contribution to helping South Sudan resolve its outstanding issues," Marsden said.
Open borders equal new opportunities
South Sudan and Sudan are linked by their reliance on oil, too. While the oil fields are largely located in landlocked South Sudan, where most people are employed in small-scale subsistence farming, the only way to get the oil out is via Sudan's pipeline and port.
Sudan receives roughly up to $11 ( €10) for each barrel of South Sudan's oil that flows through its pipeline, according to Reuters.
If South Sudan can resume its oil exports, it would be a win for both nations.
Both countries also hope for better economic relations. Sudan is suffering a serious economic crisis - rising food prices and high inflation led to the mass protests that finally pushed Bashir out of office.
As for South Sudan, it's one of the poorest countries in the world and six years of civil war have ravaged what little economic activity took place when it became independent.
"It is hoped that the borders will be opened again and trade can take place," said Marina Peter from Bread for the World.
Some voices in Sudan and South Sudan even suggesting the formation of a kind of confederation in the future, Peter said, an idea that seemed unthinkable after the two countries' messy divorce.
Whether such dream will ever become a reality depends on the strength of the new friendship between the former foes - and of course, the stability of their new governments.