With DMD32 D Compiler v2.062,

$ cat libA.d
module libA;

extern (C)
{
        struct Opaque;  // forward reference
        Opaque* getObject();
}

$ cat libB.d
module libB;

extern (C)
{
        struct Opaque;  // forward reference
        void doAction(Opaque *);
}

$ cat bug.d
import libA, libB;

int main(string[] args)
{
        auto opaque = libA.getObject();
        libB.doAction(cast(libB.Opaque *)opaque);
        return 0;
}

$ rdmd bug.dlibB.d(5): Error: struct libB.Opaque unknown size
libB.d(5): Error: struct libB.Opaque no size yet for forward reference
libB.d(5): Error: struct libB.Opaque unknown size
libB.d(5): Error: struct libB.Opaque no size yet for forward reference
libA.d(5): Error: struct libA.Opaque unknown size
libA.d(5): Error: struct libA.Opaque no size yet for forward reference
Failed: 'dmd' '-v' '-o-' 'bug.d' '-I.'
$

If we declare Opaque in a separate file and import it in libA and libB, then there is no compiler error. Also if we remove the cast in bug.d, the error message would be:

bug.d(6): Error: function libB.doAction (Opaque*) is not callable using argument types (Opaque*)
bug.d(6): Error: cannot implicitly convert expression (opaque) of type Opaque* to Opaque*

The compiler should have qualified Opaque so that it is easier to understand that it cannot convert libA.Opaque* to libB.Opaque*