No problem.

Ah, now you're talking two separate assemblies, in addition to separate
namespaces.

Right, obviously. This is not possible. You should rearrange your classes
in the assemblies (or even merge them) so that dependencies all work "in
one direction".

I don't feel I know enough about your specific situation, but a common
solution in similar situations would be to have the data access class and
a base class for the middle tier object in one assembly and let the data
access class work only with the base class. Then the middle tier class
could be in a different assembly, derived from the base class, and the
directions or dependencies would be fine. Like this:

Classes in assembly "data":

  class DataAccessObject {
    public DataAccessObject(MiddleTierObject middleTierObject) {
      // do something
    }
  }

  // Maybe this would even be an interface only? And in the real
  // world, it might not be called MiddleTierObject because the
  // data assembly really doesn't know or care about any middle
  // tier. Just to make things clear here.
  abstract class MiddleTierObject {
  }

Classes in assembly "middletier":

  class MyConcreteMiddleTierObject : MiddleTierObject {
  }

               Oliver Sturm