Multiple Blender Threads (targetting CPU and GPU ) I am using blender 2.77a on windows 10. I intend to do work in separate blender thread(modeling, texturing..small tweaks) while I put my finished frames to render. While rendering blender uses 100% GPU and only 30% CPU. 

I am facing serious lag while running another blender process, however I can run other windows applications smoothly as I still  have like 70% CPU and enough memory. 

I am sure there is possible solutions as I intend to run another blender process on CPU only. Any suggestions are welcome.
Blender uses OpenGL for both the 3D viewport and rendering. If you use GPU rendering the render engine occupies the whole GPU and AFAIK there are no settings to influence that in the current builds (even not in development builds). 

The only way to get a little more responsiveness while rendering in another instance is to switch from Cycles to Blender internal and then switch the 3D viewport from GLSL to Multitexture. This might at least help with the modeling part. 

Other software might continue to work without (or maybe a little) lag, if they don't make use of your GPU in any way. Keep in mind that even more recent versions of MS Office (since 2013) puts load on the GPU to speed up rendering as well as some audio players.