Sussex and Surrey Police are to work together on major crime investigations, forensics and firearms following approval from their police authorities.
It follows similar arrangements in Kent and Essex, and was not a precursor to a merger, the authorities said.
The collaboration will allow the neighbouring forces to work together in a "more efficient and effective way" and make savings.
Both forces need to make annual savings of at least £3.46m in total.
Surrey Police Authority chairman Peter Williams said: "Police authorities are responsible for striking a balance between providing the resources our chief constables need to keep the public safe and providing value for money to the taxpayers who fund our service.
"Collaborating on major crime investigation, forensics and firearms in Surrey and Sussex will help us to make even more - both operationally and financially - of the significant investment we make in these essential police services.
"We will continue to look for further opportunities to collaborate with our neighbouring authorities that may offer similar benefits."
Dr Laurie Bush, former chairman of the Sussex Police Authority and lead member for collaboration, added: "These areas of collaboration represent a huge step forward and will allow Sussex and Surrey to work together in an more efficient and effective way for the benefit of the communities they serve."