Have you tried the instructions in the manual?
http://e-docs.bea.com/wls/docs81/jndi/jndi.html#475689

If you have, and are having trouble with that, you probably should ask
in the WebLogic clustering newsgroup:
weblogic.developer.interest.clustering

If i get you right you have two separate domains trying to talk to each
other. If so your problem is not clustering. You will probably have to
specify the correct host for communication in the phase of getting a
JNDI Context.

To lookup an EJB from the other cluster you might do the following...

InitialContext cts = new InitialContext() ;

Thats the problem. BEA assumes you mean the Context of the cluster this
piece of code runs in. If you want the other cluster you need to
specifiy the cluster by giving JNDI Parameters when creating the context.

You might take a look here
http://e-docs.bea.com/wls/docs81/javadocs/weblogic/jndi/WLInitialContextFactory.html

where you will find:
   Hashtable env = new Hashtable(5);
   env.put(Context.INITIAL_CONTEXT_FACTORY,
           "weblogic.jndi.WLInitialContextFactory");
   env.put(Context.PROVIDER_URL,
           "t3://weblogicServer:7001");
   Context ctx = new InitialContext(env);

hope this solves your problem.

regards
Volker