@sim_157 hi!
You can implement it via two ways: the dual-link backup and N+1 backup.
The dual-link backup allows two ACs on an AC + Fit AP network to manage the APs simultaneously.
N+1 backup uses one standby AC to provide backup services for multiple ACs on an AC + Fit AP network.
In the dual-link backup mode, each active AC has an independent standby AC. Unlike the dual-link backup, N+1 backup uses a standby AC to provide backup services for multiple ACs, which reduces device purchase costs.
For more details and examples, please refer to: