Not possible. you cant add 2 blend modes. dont ask me why, just i know you cant, and you will never be able to cause webgl does not work that way. it can only apply one blend mode, same photoshop does, you can apply one blend then if you want you can rasterize the image then apply another one but the old blend wont be there. so the long answer wold be "Nope"
ps. see if you have a option to fallback blend mode, and set that to additive, but that only happened only when a effect is added (hue saturation etc)
i remember doing a old project, with a girl in a mirror, and i was using the same source atop blend trick, but for some reason its not working anymore, unless the bottom or top source is on additive, and even then its going to look bad in the new C2 version atleast, the version i used for girl in mirror and worked, was back around 167 or 180 i think but i cant remember was months ago, maybe a year.