Combine multiple GL draw modes or contexts?

Hi all,

I make anaglyph graphics in Jitter GL. I use drawmode 3 5, to add colors on top of one another (so that a shape's red view plus its blue view = white shape). The drawback, however, is that the shape's brightness determines its opacity: dark shapes are transparent, moderately bright shapes are translucent, and only bright shapes/textures are opaque.

I'm looking for a way around that. I hope that something like the following is possible: take a shape, sum the red and blue versions (drawmode 3 5) offscreen, and then layer that result onto the scene (with regular drawmode). Is such a thing doable, i.e. are separate rendering contexts available that can render separately and then be combined on screen via a separate drawmode?

Thanks for any leads!

Dan