I you need to serve two or more subnets on one physical interface using isc-dhcpd just wrap subnet declarations in shared-network clause:

shared-network interface {
    subnet 192.168.0.0 netmask 255.255.255.0 {
        option domain-name "subnet1.tld";
        option domain-name-servers 192.168.0.1;
        option routers 192.168.0.1
        option broadcast-address 192.168.0.255;
    }
    subnet 192.168.1.0 netmask 255.255.255.0 {
        option domain-name "subnet2.tld";
        option domain-name-servers 192.168.1.1;
        option routers 192.168.1.1
        option broadcast-address 192.168.1.255;
    }
}

Replace interface with real interface name.