The Avon and Somerset and Wiltshire forces will form a "strategic alliance" in areas including IT, HR and administration.
Wiltshire's Police and Crime Commissioner (PCC) said ruling out job cuts would be "foolish" but that the move did not herald a full merger.
Avon and Somerset's PCC said some sharing "could protect local policing".
Wiltshire and Swindon's Conservative PCC Angus Macpherson said he would be "very foolish" to rule out job cuts when millions of pounds in savings had to be found.
But he said it was "absolutely not" the forerunner to a full merger.
"There will be changes as we move forward...[but] I want to preserve the officers who respond to calls in Wiltshire and Swindon," Mr Macpherson said.
"That's what I'm absolutely committed to doing."
His Avon and Somerset counterpart, Sue Mountstevens, an Independent, said: "My job as PCC is that when we have cuts, [we] minimise the impact on local communities.
"By doing this collaboration...by saving money in back-office, in specialist functions,... we're trying to lessen the impact on our frontline."
On a visit to Somerset, the Prime Minister welcomed news that Avon and Somerset and Wiltshire police were going to work more closely together.
David Cameron said: "I would praise the police because they have had to take difficult decisions over the last few years.
"But crime in Somerset has fallen quite markedly, so the police have demonstrated that they can do more effectively with less and increase the percentage of officers on the front line."
Mike White, from the Wiltshire Police Federation, said "We do have concerns as to how this will effect our members and the impact the inevitable cuts in staffing that this will lead to.
"Ultimately cuts have consequences.  We can only hope that this is a long-standing alliance and not the first step towards the disappearance of a historic and proud police force."
Since 2010, the 43 police forces in England and Wales have seen their budgets reduced by 20%.
Recently, the Avon and Somerset force sold off some of its police stations.
It has had to save Â£47m over the past four years and more cuts are expected.
Wiltshire Police needs to cut another Â£12m over the next few years.
Two police forces in the West Country are to merge some of their back-office functions, it has been announced.