This can be easily done in the Selected event. The following code assumes you have two ImgXCtrl objects on your form named ImgXCtrl1 and ImgXCtrl2.
Private Sub ImgXCtrl1_Selected(ByVal Left As Long, ByVal Top As Long, ByVal Width As Long, ByVal Height As Long)
' Stop display changes.
ImgXCtrl2.AutoUpdate = False
' Copy the image.
' Crop it.
ImgXCtrl2.Effects.Crop Left, Top, Width, Height
' Refresh the display.
ImgXCtrl2.AutoUpdate = True
Now just set the ImgXCtrl1.MouseTool = ixmtSelect and select an area of the loaded image.