Visual C++ :: Passing DDX Control Variables Between Dialogs (Parent->Child)

I have a main dialog which has (DDX?) controls for the user. eg a slider, and a combo box.

Previously, an image adjusted by these controls was in the same dialog.

I need to put this image in a New, separate dialog but it still must be controlled by the slider and combo box on the main dialog.

My question is how can I pass the control's variables between the dialogs? I have, say,

Code:
//MainDlg.cpp
DDX_Control(pDX, IDC_ComboBrightness, m_Brightness
I was told that I could do something like:

Code:
//ImageDlg.h
public:
ImageDlg(CWnd* pParent = NULL, CComboBox m_Brightness); // standard constructor

But I get errors including: Missing default parameter for parameter 2

I also need to pass the image array, is that perhaps the same method?