The two police authorities have signed an agreement to form one "information management unit" that would save Â£600,000 a year.
The Thames Valley Police Federation said this raises questions about the possible amalgamation of whole forces.
A police spokesperson said the unit would improve customer service.
"Some services provided directly to the public will be improved by having a larger pool of staff to process their enquiries," he said.
The unit, which will share and manage police data across the two forces, will be based in existing locations across Hampshire and Thames Valley.
The spokesperson said: "The creation of the joint unit will reduce the total number of posts required to deliver information management for the two forces.
"However, existing vacancies have been held open and it is anticipated that the new unit will be created with few, if any, redundancies."
He added that the savings across the two forces would be met through "a shared management structure and streamlined processes".
Graham Smith, from the Thames Valley Police Federation, said: "On a wider issue with so much collaboration taking place around the country there comes a point where questions have to be asked about the possibility of amalgamation of forces."
Hampshire and Thames Valley Police have merged two units in a cost-saving measure that could lead to redundancies, the forces have said.