The mobile module has a couple of options, a theme on the same site, or a different site. We're opting for the "different site" option. I've attached a screenshot of our mobile tab.
Two hosting plans isn't really what we want, as I don't think we can make a separate hosting account for a subdomain. I've put the whole mobile site into a "mobile" folder, and want to have the m. subdomain redirect there.
Does the two sites share common pages and content? That is why with theming you can have a different looking site, but same content etc.
Did you just copy entire silverstripe install folder into sub folder /mobile? If so this wont work as you would need two different hosting plans. Hence why mobile module and themes allows you to have two sites, but sharing same code and content etc.