The contract follows a 10-year £13m grounds-upkeep and waste-services contract secured by the Landscape Group with the two authorities last week.
"Hastings and Rother councils have joined forces to secure yet another major contract and save money in the process," said a spokesman for Gristwood & Toms.
"As a result of putting the tender out as a partnership, the contract has resulted in reduced costs for tree work for both councils."
The deal also allowed for AmicusHorizon housing association to join the contract and take advantage of more competitive tree work rates, the spokesman added.
Hastings head of environment Emily Westley said: "We have shown that when local authorities form partnerships to procure contracts we can get a better deal for the tax payer while maintaining excellent standards."