You can sort of do that now with Retroarch. It has the options to combine more than one filtering shader to an Emulator Core. Since it uses shaders to implement the filtering, you could take a look at the shader code.

 

Here is the source code to the render plugins:

http://www.emucr.com/2013/04/kega-fusion-plugins-20130427.html

I'm sure someone can chime in if there is a source code repository anywhere for this.

Also, with a recent build of VBA-M, if you go into Video > Render API, you can enable bilinear filtering there for both DX9 and OpenGL. Then, enable 2xSAI through the filter plugin and you will have bilinear + 2xSAI. Kega Fusion has a similar option in Video > Filtered. Although this may not be exactly what you want.

 

there is no point to this, atleast on vba-ms end

Bilinear is already possible via the renderer settings for d3d and opengl.

 

Thanks for your support.
I tried already the bilinear D3D or opengl option in vba-m together with the filtering.
But as soon as the filtering is activated the bilinear filtering is deactivated.
Same for Kega Fusion. You can easily compare the resulting images and you will clearly see the difference. That's why I wanted to create such a filter.

The only emulators which can combine bilinear filtering with other filters like 2xsai are snes9x and nestopia. Retroarchs shaders can be stacked as well.
All other emulators (including vba-m and fusion) can just activate the one or the other filter effect.