two masters for the same zone

[srg]

Hello:

I need the following scenario:

1. two or more nameservers (ns1, ns2, ...) MASTERS of the same zone, ie, 
example.com.
2. one or more clients do ddns updates to the nameservers in (1). Those 
updates will be done sometimes to ns1 and other times to ns2.
Here comes de problem:
3. If a client registers it's name (client1) in ns1's example.com. zone 
then I need that the name client1.example.com. to be resolved in ns1 AND 
IN ns2.
4. The same if the client registers in ns2's example.com. zone (the name 
MUST be resolved in ns1 and in ns2).

In other words, I need that the ddns name registrartion (using nsupdate 
or any other ddns client) can be done in ns1 and in ns2, and, that the 
registered name will resolve in BOTH n's no matter the one used to register.

I have been done a similar configuration but configuring ns2 as a SLAVE 
(including to do the updates in the slave and then the salve forward the 
update request to the master), but the problem is that if ns1 goes down 
then no more clients can be registered.

Thanks and best regards