Render Splitscreen to RenderTexture

I'm trying to capture the screen to a texture (primarily for supersampling to compensate for large pixels from using distortion effects) when doing splitscreen rendering; however the camera's viewport rect does not seem to be taken into account when rendering to a texture.

How shall I best go about setting things up to render both views into the same texture? Are there blitting solutions fast enough for realtime capture to allow each camera to have a separate view and then I manually stitch them together?

Thanks for any help.