I have signed the applet. And I can already read and write the client side without changing the .java.policy file.
The client can also connect to the server which it downloaded the applet from.
So, no problem with signed applet or communication between applet client and the server. The only problem I can think is that clients are not allowed to connect to more than one server even if the signed applet is used. :-(
Are you sure clients can connect to more than one server by using signed applet?