Divide the pool of IP addresses so that each server is responsible for a range of addresses and there is no overlap of responsibility.
Choose NIS+ as your data store, if available. If not, choose files and specify a shared directory for the absolute path to the data store.
Configure each server separately so that address ownership is allocated correctly and that server-based macros can be automatically created.
Set up the servers to scan the options and macros in the dhcptab file at specified intervals so they each are using the latest information. You can do this by using the -t option with in.dhcpd(1M).
Be sure that all clients can access all DHCP servers so that the servers can support one another. If a client has a valid IP address lease, and is either trying to verify its configuration or extend the lease, but the server owning the client's address is not reachable, another server can respond to the client after the client has attempted to contact the primary server for 20 seconds. If a client requests a specific address, and the server owning that address is not available, one of the other servers handles the request. The client receives a different address from the one requested.