What you appear to have now is an Employment Visa. This permits you only to work for the company which sponsored that visa. If you are also working for another company without the Immigration Department's permission then you have been breaking the terms of your Visa, and there is a possibility that it could be withdrawn and you asked to leave. So you need to be careful.
What you describe as a "business visa" is what is officially called an Investment Visa. In order to get an Investment Visa you have to show credible business plans for your business, and that it will add value to the Hong Kong economy, preferably by employing local permanent residents. Immigration is not interested in how much you will earn except in that you must be able to demonstrate that you have sufficient money to support yourself at a reasonable level and will not become a burden on the taxpayer.
The application form for an Investment Visa is the same one as for your Employment Visa, although you need to complete it slightly differently and supply different supporting material.
The form is
here, and the Guidance Notes are
here.