Can I render video as we all create video to render with place holder simultaneously in After effect
the meaning could be simple, but the way you phrased your questions makes is difficult for me to understand what you mean exactly. you can encode your file in AME while you still work in Ae and you can open multiple instances of Ae in the same computer if that's your question. check this tutorial at 13:16 Rendering Workflow CC 2015.2 - After Effects vs. Adobe Media Encoder - YouTube
One more question, right now when ever we render any video through After effect CS6 script it automatically open After effect rendering software & the rendering is done.
can't we do the process in background with the help of some plugin so that we can use AE for other purposes (like creating video to render.....)
if you set you Ae to -m like the tutorial suggests, you can open another instance of Ae and do what ever you what while another instance is rendering. if you need more background rendering options I would recommend you examine this: BG Renderer Pro - aescripts + aeplugins - aescripts.com