Is it permissible to combine working for a contracting company with owning a company that supplies equipment to that same company, and what is the ruling on profits previously earned?

There is no objection to establishing a company with others to provide equipment for your workplace. However, if the decision to contract with suppliers is part of your job, then it is not permissible for you to contract with your company unless you inform the officials, obtain their permission, and ensure the best interest of your workplace. But if the contracting matter is not in your hands, there is no objection to your company submitting an offer to your workplace. It is not forbidden for you to combine your job with your company if your work in your company is outside official working hours and does not affect your primary job.