You could perhaps draw to the backbuffer, copy the backbuffer to the accumulation buffer, draw your second image to the backbuffer then multiple back the accumulation buffer content?
What are you trying to do exactly? Some kind of parallax?
I wonder if drawing to the backbuffer's alpha channel would help with filtering in/out the second graphic. You could draw the first image, then draw alpha images to the backbuffer's alpha channel to show how much of subsequent images you want to show, then set the rendering/blend mode of OpenGL so that it multiplies by the alpha value, then draw all the second layer of images in that mode so they only draw in the intensities of the alpha buffer.
What are you trying to do exactly? Some kind of parallax?
I wonder if drawing to the backbuffer's alpha channel would help with filtering in/out the second graphic. You could draw the first image, then draw alpha images to the backbuffer's alpha channel to show how much of subsequent images you want to show, then set the rendering/blend mode of OpenGL so that it multiplies by the alpha value, then draw all the second layer of images in that mode so they only draw in the intensities of the alpha buffer.