This intruction is based on my experience. You may find a different way. The steps are as follows:
Go to your CPanel,
Open Domain > SubDomain,
Fill in the subdomain input filed, such as: mobile (my second store will be http://opencart.techphpexpert.com),
Look at Directory Root input field, CPanel will fill it automatically. Don’t use it,.. Replace with your main store’s OpenCart installation folder. In my case /public_html/shop. Click Create.
In OpenCart Administration
Go to System > Settings,
Click Create A New Store button,
Fill in all required fields including the logo, favicon, etc.
In Store URL, fill it with http://opencart.techphpexpert.com,
Don’t forget to edit Category, Product, Manufacture, and Information Pages that you want to be displayed in your second store.
Important Notice : In your configuration file there is “/” added in selecting store query. so you must have to remove that for above to work.