Setup reservations for the specific hosts using the dhcp-host option. Use tag options to flag those for a special set of options.
This is completely un-tested, but I suspect your config might look something like this. Check the man page for full details.
dhcp-host=00:15:99:27:f8:98,set:specialhosts,192.168.32.20
dhcp-host=00:51:99:72:8f:89,set:specialhosts,192.168.32.21
dhcp-option=tag:specialhosts,option:dns-server,8.8.8.8,8.8.4.4