You are closely to the true, but you just need to wait a little before we launch the official tutorial for that :) Actually I have teamed up with guys from baby.lv forums
who were interested in mobile version, and currently, it's almost developed. Last testing is performed. You can see for yourself (be afraid of the Cyrillic text): m.baby.lv
It is of course not
about having two identical copies of the software. This is actually lame. You will kill yourself later on upgrades and routine work, if you have multiple copies. Actually, multiple copies solution is for those who want to install it once and forget about it. It's not practical.
solution is to run both versions on the same core, and dynamically allow the script to use the alternative set of templates (located in [templates] folder) and CSS file (bb_mobile_style.css) depending on which domain is loaded. Mobile subdomain should point to the same folder as the root domain.
Duplicated content may be set defining $metaRobots var (NOINDEX,NOFOLLOW tags for all pages) or in robots.txt. The password protection will also work for regular visitors, it's not comfortable for them, to enter password for accessing the content, specially on mobile phones ;)