Database Mirroring Using Certificate? (two Principles And One Mirror)

Hi, all.

here is the situation.

we designated one server as a mirrored server. this server will host all mirrored databases from different principles in different server using certificate.

1) Server A (Principle) Server B(Mirrored)
2) Server C (Principle) Server B(Mirrored)

I was able to set up mirroring between A and B. But unable to set up mirroring between C and B.
My question is "Can I grant connect on endpoint to two different host logins?"

For example, on the server B
I created two different logins for Server A and C in order to assign different certificates to two principle servers.

For Server A.
create login server_A_login with password = 'djkkajnxks';
go
create user server_A_user for login server_A_login
go
create certificate server_A_cert
authorization Server_A_user
from file = 'c:certificateserver_a_cert.cer'
go

GRANT CONNECT ON ENDPOINT::Endpoint_Mirroring TO [server_a_login];
GO


For Server C.
create login server_C_login with password = 'djkkajnxks';
go
create user server_C_user for login server_c_login
go
create certificate server_c_cert
authorization Server_c_user
from file = 'c:certificateserver_c_cert.cer'
go

GRANT CONNECT ON ENDPOINT::Endpoint_Mirroring TO [server_c_login];
GO

when I set up like this and try to start mirroring, I am getting error from server c saying, "the network tcp:serverb.corp.com can not be reached or does not exist"

But mirroring between server a and b is OK. I set up them first, then between c and b.

any ideas?

thanks,