I am still quite unsure about the requirement here.
pp.Presentation PPT1 = pptApp.Presentations["Presentation1"];
pp.Presentation PPT2 = pptApp.Presentations["Presentation2"];
pp.Slide ppt2ActiveSlide = PPT2.Slides[pptApp.ActiveWindow.Selection.SlideRange.SlideIndex];
foreach(pp.Slide slide in PPT1.Slides)
{
foreach(pp.Shape shp in slide.Shapes)
{
shp.Copy();
ppt2ActiveSlide.Shapes.Paste();
}
}
Hope it helped.