Registration-caching parallel forking

Hi guys

today i found a surprising problem while i was doing some tests to a customer site  , this is the scenario :

i registered 2 endpoint with same AOR , both endpoint register correctly on the registrar/proxy server with different cookie applied by the sbc .

The problem is that when i call this AOR the proxy/registrar send both INVITE to sbc but only one is forwarded by the sbc , the other is answered back toward the proxy with a 480 No routed found so only one phone rings , i'd expect that both ring .

Do i need to add some strange option to enable parallel forking ?