The ejb specification requires that both entities be in the same ejb-jar file. You will have to merge your two ejb-jar files.
what this communication entity is required by many entity
whose requirement many not be know now then all such entity have to be clubed in on ejb-jar.xml file this will make the ejb-jar.xml file to big to maintain.
Yes. It is required by the EJB specification. The spec committee is currently working on 2.1, so you could attempt to get them to allow relationships between entities in different jars.
You could code your relationship by hand (at least a unidirectional one). Just write a base class for all EJB's that shall have this relationship. This base class needs a CMP field with the type of your relationship-dest's PK. Then you can code a get / set method for your related EJB using JNDI. To keep things flexible the JNDI name of your relationship-dest should be submitted to the EJB using an env-entry.
I will try that one.