DDNS conflict with zone update

Hi
I got a configuration of two nameserver a master (192.168.0.5) and a
slave (192.168.0.1). The slave nameserver was first configured to ddns
with dhcp3-server (it was working flawless). Then I added the master
nameserver and the zone updates between master and slave would only
work, if ddns was comment out. I can paste both full configurations if
necessary, but the master/slave setup is pretty straight forward.

named.conf.locale @ Master-nameserver:
zone "utysket.dk" {
type master;
file "utysket.dk.zone";
notify yes;
allow-transfer { key MasterSlave; };
};

named.conf.locale @ Slave-nameserver:
zone "utysket.dk" {
type slave;
masters { 192.168.0.5; };
allow-notify { 192.168.0.5; };
# allow-update { key "ddns-key"; };
file "utysket.dk.zone";
notify yes;
};

Is it conflicting to have two sources updating the zone-file? Another
thing regarding ddns and my setup. DDNS increases the serial in the
slaves zone file. So if there is enough dhcp changes the serial can
exceed the serial of the master name-servers zone file. Is it just a
matter of setting the serial high enough or it is generally a bad idea
to have ddns on only the slave?

/Lars