ambiguous conversions from 'class ATL::CComObject<class CTstConnect> *' to 'struct IUnknown *'

I am doing some testing with ATL Connection Points.  I have created a project that has 2 ATL Objects, ObjA and ObjB (Simple Objects created through the ATL Object Wizard) - each of these support Connection points.  IObjA has one method foo() and IObjAEvents() has one method foocb().  foo() just pops up a MessageBox and then calls Fire_foocb(). This works fine and proves that the connection point between my App and ObjA works ok.

So I want now to also get ObjB events so I thought that I could just extend my class that implements the IObjAEvents interface to also implement IObjBEvents.
TstConnect.h that contains my implemntation class CTstConnect is listed below. I thought that by adding COM_INTERFACE_ENTRY(_IObjBEvents) to the COM_MAP and adding _IObjBEvents to the list of base classes that CTstConnect derives from then I would be able to implement the IObjBEvents interface in CTstConnect.  However, when I build I get the error
error C2594: 'argument' : ambiguous conversions from 'class ATL::CComObject<class CTstConnect> *' to 'struct IUnknown *' for the line:-
    CComPtr<IUnknown> ptrEventsUnk = ptrTstConnect; in my OnButton1 event
Is this basically the wrong approach - do I need to create a new class for each event interface that I want to implement ?

void CMymfcclientDlg::OnButton1()
{
    HRESULT hRes = CoInitialize(NULL);
    _Module.Init(ObjectMap, AfxGetInstanceHandle());
    CComPtr<IObjA> ptrObjA;
    hRes = CoCreateInstance (CLSID_ObjA,NULL,CLSCTX_SERVER,IID_IObjA,(void **) &ptrObjA);
   
    CComObject<CTstConnect>* ptrTstConnect = new CComObject<CTstConnect>;
   
    CComPtr<IUnknown> ptrEventsUnk = ptrTstConnect;

    DWORD dwCookie;
    hRes = AtlAdvise(ptrObjA, ptrEventsUnk, DIID__IObjAEvents, &dwCookie);
    ptrObjA->foo();
    AtlUnadvise (ptrObjA, DIID__IObjAEvents, dwCookie);
    CoUninitialize();
}

TstConnect.h
------------
#include <atlbase.h>
CComModule _Module;
#include <atlcom.h>
#include <atlimpl.cpp>

BEGIN_OBJECT_MAP(ObjectMap)
END_OBJECT_MAP()

#include "..\callbacktest.h"
#include "..\callbacktest_i.c"

class COMModule
{
public:
    COMModule()
    {
        CoInitialize(0);
    }
    ~COMModule()
    {
        CoUninitialize();
    }
};

COMModule gModule;

class CTstConnect :
    public CComObjectRoot,
    public _IObjAEvents,
//  public _IObjBEvents
{      
public:
    CTstConnect(){ }
    ~CTstConnect(){ }

    BEGIN_COM_MAP(CTstConnect)
        COM_INTERFACE_ENTRY(_IObjAEvents)
//      COM_INTERFACE_ENTRY(_IObjBEvents)
    END_COM_MAP()

public:
    STDMETHODIMP GetTypeInfoCount(UINT*)
    {
        return E_NOTIMPL;
    }

    STDMETHODIMP GetTypeInfo( UINT iTinfo, LCID lcid, ITypeInfo **ppTInfo)
    {
        return E_NOTIMPL;
    }

    STDMETHODIMP GetIDsOfNames( REFIID riid, LPOLESTR *rgszNames, UINT cNAMES,      LCID lcid, DISPID *rgDispId)
    {
        return E_NOTIMPL;
    }

    STDMETHODIMP Invoke ( DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS *pDispParams,
                          VARIANT *pVarResult, EXCEPINFO *pExcepInfo, UINT *puArgErr)
    {
        switch (dispIdMember)
        {
            case 0x1:
                if (pDispParams->cArgs != 0)
                    return DISP_E_BADPARAMCOUNT;
                if (pDispParams->cNamedArgs)
                    return DISP_E_NONAMEDARGS;

                foocb();
                break;

            default:
                MessageBox( 0, "Error", "Test", MB_OK);
                break;
      }
      return S_OK;
    }

    STDMETHODIMP foocb()
    {
        MessageBox(  0,  "foocb",  "Test",  MB_OK  );
        return S_OK;
    }
};