QUESTION: How to implement Split Horizon DNS with dnsmasq?

[Patrick M. Hausen]

You did split horizon with Unbound. Just keep doing it the same way. Split horizon and dynamic leases are in no way related.

[klinebau]

I assume you have hosts registered in dnsmasq and you want to reference those using a local dns name.  In this case, you would set up query forwarding in Unbound to point queries with this domain to dnsmasq. This documentation contains an example: (https://docs.opnsense.org/manual/dnsmasq.html#dhcpv4-with-dns-registration).  I've been on dnsmasq for a while now and don't remember how this was done with ISC, but it might have been automatic before.

If my assumption is false and you are trying to do something else, then I don't know.