Asterisk Realtime & Multiple Servers


I have tried two servers using the same database server and can register to either, however if I try to make a call from a server to a phone that has registered on the other server the call fails.

Why does this happen? Does each asterisk box store additional information on the registered devices, instead of placing this into the database?

I know I can implement this using OpenSER as a registrar server however I wondered if it was possible without.

Thanks in advance,