Jeremy Clarke : Both halves of post-split Sudan need each other

The north's Khartoum feels secure and ordered, but riven with anxiety about what will happen after the departure of the oil-producing south. About 750 miles away, the southern boomtown Juba is lawless and chaotic, but buoyant with optimism about its future.

The sharp inequality between the relative development of the North, at least in its cities, and the impoverished South was one of the grievances that drove southern rebels in decades of civil war against Khartoum.

Juba's dusty streets, in contrast to Khartoum's eight-lane Chinese-built highways, highlight how far the South still needs to go to build a nation, essentially from scratch.

Southerners chose to secede in January, in a vote which was part of a 2005 peace deal that ended the country's civil war.

A mishmash of former rebels now polices the South, which is roughly the size of France. Most lack formal training, and a dearth of powerful institutions means the state's writ often ends where the barrel of an officer's gun begins.

Uniforms and cars give the force a veneer of order, but the country-in-waiting still lacks clear laws.

The legal vacuum fosters an air of hedonism unthinkable in Khartoum. Cars zip down dirt roads without concern for speed limits and residents swig Kenyan lagers openly in outdoor cafes.

In the north, even the water pipes that used to bubble in roadside cafes were outlawed this year. Drinking heavily sugared tea is the biggest vice commonly accepted in public.

Many northerners are nervous about the ubiquitous secret police, refusing to talk politics over the phone.

A ride through Juba's potholed streets and muddy lanes shows how much work remains to be done in what will be considered one of the world's least-developed countries.

There are few buildings higher than two storeys in the city, which is rapidly expanding.

Many hotels use tents or pre-fabs. Government meetings are often interrupted by electricity blackouts. Seated at the confluence of the Blue and White Niles, Khartoum still has miles of dirt roads and snarling traffic. Along the Nile, a smattering of self-consciously modern glass and steel towers and British colonial-era red brick compounds, now housing ministries and museums, give the city an air of history and wealth lacking in its southern counterpart.

Outside downtown's grid-like layout - it is said the former British masters modelled it on the Union Flag - designer restaurants and cafes catering to wealthy Sudanese and foreign workers have sprung up.

The two capitals are often distinguished by their Arab and African characters, but such generalisations deny the complexity and diversity of both.Underneath the surface there are plenty of similarities that suggest the two will remain to some extent socially and economically interwoven even after the split becomes official.

Both cities are relatively new, set up by the invading Turkish-Egyptian army in the 19th century. In both, Arabic is the language used across a range of tribes and ethnicities.

Oil is the lifeblood of both economies and their industries are deeply intertwined. Most of the fields are in the South but nearly all the ports, refineries and pipelines are in the North.

Both also face widespread complaints of corruption. Media in the North became increasingly vocal in criticising the endemic nature of the problem after revolts in neighbouring countries.

In Juba, corruption reared its head as guerrilla fighters took over a government with almost 1 billion a year at its disposal.

On the town's main new road, massive new homes for ministers have appeared, complete with parking for shining new 4x4s or even Hummers.

Locals curse the cars as they power down the unofficial "government lane" which runs through the middle of Juba's streets.