They are separated by little more than a motorway and a golf course, but a growing urban sprawl has in places seen them almost merge.
Now advocates of better local government say its time politics reflected a creeping geographical reality.
They want Southampton City Council to expand its borders and welcome in its smaller town neighbour of Eastleigh.
The controversial move would create a super-unitary, similar to the metropolitan council's of Manchester and Liverpool.
It would do away with the two-tier structure of borough and county council that Eastleigh residents have come to know since the 1970s.
Champions say structural change is needed to meet government calls for increased efficiency, better services and value for taxpayers, and stronger accountability and leadership from council leaders.
Opponents say greater collaboration is the answer and have rebuffed recent invitations from the government to create new or enlarged unitary authorities in Hampshire.
However, a Bill currently trundling through Parliament will give the secretary of state powers to impose change if necessary, following consultation.
It is expected to get Royal Assent to pass into law in the autumn.
Sixteen bids for unitary status elsewhere in the country have been approved and will now go out to public consultation.
Local government expert David Kett, who has lectured on the subject for more than 30 years, reckons a Greater Southampton authority is long overdue.
"This debate has been going on since 1974 when there was a proposal for a Solent city," he said, pointing to the Redcliffe-Maud review of 1969 and subsequent shake-up.
Mr Kett, a critic of two-tier government, says much council work is duplicated and overlaps. This makes it less answerable because many residents are confused about who is responsible for what.
He said that when Southampton and Portsmouth became unitary authorities in 1997 the boundaries were drawn too tightly, ignoring the reality of a "conurbation" on the ground.
Areas such as Fareham and Eastleigh have now practically merged with their city neighbours with in-fill developments, he said.
"We now have in essence a metropolitan district. We should accept that and administrate it as such," Mr Kett said.
He proposes two authorities in south Hampshire, a Portsmouth Harbour authority covering Havant, Portsmouth, Fareham and Gosport, and a Solent Authority, bringing together parts of New Forest, Southampton, and Eastleigh.
"The big problem is snobbery," he said, admitting "It's probably on the back-burner for the next 15 years or so."
Eastleigh survived the last local government shake-up in the mid 1990s.
Instead, the principal recommendation of the Local Government Commission tasked with the job - to create unitary authorities out of Portsmouth and Southampton only - was accepted by the government.
A second option was to replace the Hampshire County Council and district councils throughout the whole of Hampshire with seven unitary councils, including an Eastleigh and Southampton merger.
It failed to win enough public support among the almost 3,000 representations made.
About 40 per cent wanted the present two-tier arrangements to continue, largely because of Hampshire's particular community identities, while one third supported some form of unitary solution.
Mr Kett said both Conservatives and Labour backed the unitary authority model but the Tories lacked the political will to impose it on their English shire heartlands during the 1990s, although they did so in Wales.
Southampton City Council's Conservative deputy group leader Royston Smith, is one politician open to exploring the Eastleigh merger idea.
He said: "I wouldn't dismiss it out of hand. I'm quite relaxed about a super unitary.
"In my opinion there are far too many tiers to local government.
"There are bits of Totton, Romsey and Eastleigh that are geographically close to Southampton.
"There probably is some mileage in it. We all know what their post code is."
Cllr Smith said a merger would also tidy up some quirks such as Southampton International Airport being in Eastleigh.
Labour's Matt Stevens is another Southampton politician who believes single-tier government is easier for people to understand and would boost efficiencies.
"Bigger boundaries for the city might mean taking in parts of Netley, Eastleigh and Totton", Cllr Stevens said.
He sees Hampshire's future as four unitary authorities.
However, many local politicians are hostile to the idea of losing Hampshire's two-tier structure.
The proposal recently sparked a war or words in Eastleigh's council chamber, with Liberal Democrats accusing Tories of scaremongering for political gain.
A motion was even passed deploring Conservative claims Southampton City Council had "plans to submerge Eastleigh Borough into a greater Southampton."
Eastleigh Council leader Keith House said bolting on part of Eastleigh to Southampton would be "deeply unsatisfactory".
Eastleigh residents would feel like they have been taken over, he said.
Mr House was not complacent about the existing two-tier arrangements, which should be made to work better.
Fareham Council leader Sean Woodward, who heads the Partnership for Urban South Hampshire (PUSH), a group of eleven local authorities working together to map out the future of the region for the next 20 years, said there was no appetite for change.
"All that seems to happen is increased cost and very little improvement in service," he said.
"That thread runs throughout councils in Hampshire.
"We are more interested in getting on with the job.
"Restructuring is the last thing we want to do. It just diverts attention."
He said the PUSH model of closer working between south Hampshire councils had been held up as a model alternative by the government.
Nick Goulder, director of the Hampshire's Local Government Association, a lobby group which rep resents town halls, said community identity was fiercely guarded in Hampshire "People in Eastleigh are very keen not to be part of Southampton," he said.
Mr Goulder admitted there may be some minor boundary changes in the future but not "massive re-organisation".
"Most politicians think it's not worth the hassle," he said.
"At the end of the day we have to make a decision as to whether the cost of reorganising are worth the benefits."
Estimates in the last review said it would take six years to recoup transition costs.
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