Leema, without the description of a domain model and a share on the MF it is almost impossible for "us" to give you a proper answer.
It helps knowing stuff as: is there a relation between the object? Do they have a shared relation with an other object we could use? Do the share a field or enum?
Your issues might be minor or very complex, but the question does not reflect it.
Instead of commenting on your own questions, consider editing the post with additional information..... Where and when is it giving that error? What kind of retrieve are you doing on the second object? What are you trying to achieve ?