Ok, I ended up back at this thread because I need a mobile site now.
I still don't understand what my wrong thinking is. iusemodx said that you don't need two templates but seemed to disagree with my thinking but not my code advice, which he seemed to agree with tacitly.
However I still didn't know how to build the site, til I watched this video:
http://www.freshtilledsoil.com/how-to-convert-a-fixed-width-to-a-responsive-website/
Which is from the google search iusemodx gave us heh, thanks a lot it can work!