dynamic_cast does not work across modules with multiple inheritance (VC2005)

I have a project which has been running well on VC6 for years now, but
now more and more reasons are telling me it's time to upgrade. The
system consists of a largish number of dynamically loaded DLLs which
are accessed in some kind of COM fashion.

I have now ported a core set of these but run into a big problem: RTTI
does not work correctly (=the way I want it to) between DLLs anymore,
which it did in VC6.

My setup is this:
class IBase
{
public:
    virtual Foo() = 0;
};

class IDerivedOne : virtual public IBase
{
public:
    virtual Bar() = 0;
};

class IDerivedTwo : virtual public IBase
{
public:
    virtual Goo() = 0;
};

class Test : public IDerivedOne, public IDerivedTwo
{
public:
    Foo();
    Bar();
    Goo();
};

What I then do is load a module (could be the same executable) where
Test is implemented, get a pointer to a new instance of the object (as
an IBase*) through a class factory and then do a dynamic_cast to
IDerivedOne*. However, the dynamic_cast returns NULL and I can't get it
to understand what I want.

Is this the way it should be or should it give me the pointer I want?
reinterpret_cast does not work either, as a cast to IDerivedOne would
really give me IBase disguised as IDerivedOne, to disastrous results.

Does anyone know how to deal with this? Or do you need more code to see
what's going on?

Thanks for any input