All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Your question included some matters among which is the ruling on you taking a commission from the buyer and from the seller for being an agent between them while none of them knows the commission that the other gives you; this is permissible because it is lawful to take a commission from both parties of the contract or from one of them according to the custom or the agreement with the buyer and with the seller; however, it is a condition that the one from whom you take the commission knows about you taking it.
The second matter is your statement that you work for one of the companies but at the same time you have a business on your own and this is not permissible if doing that is during your working hours of the company because the time of the employee during the working hours is exclusively for the employer and it is not permissible for the employee to allocate some time of it on his own behalf without the permission of the employer. But if your work on your ownbehalf is outside the working hours of the company you work for, then it is permissible for you to do so as long as you do not use your profession and your position in the company for your own benefit referring some of the customers of the company to you for example, as there is no doubt that this is forbidden and it is an act of treachery.
Allaah Knows best.