It all depends on what software (forum, store, etc.) you're using, and whether it's built to properly coordinate between multiple sites using the same database. It's certainly possible, but the two separate sites need to know that they don't necessarily fully "own" the application.
By the way, it's not always necessary to create a separate website for mobile. Some applications can switch themes if they detect a mobile user, and others are designed to be "responsive", adjusting their content and layout based on probable bandwidth, the size of the display, and its capabilities. You might look into those areas before trying to run a separate site for mobile users.
i get your idea but i am not create a new site , just making the exact same copy on m.mydomain.com but in mobile version, if you have better idea, please share them with me.
and can you please show me the steps?