Is there any Sharia-related doubt in my receiving two different commissions, one from the client and one from the company, as a sales intermediary, and am I required to know the payment method agreed upon between them to avoid such doubt?

The scenario mentioned in the question falls under the permissible category of brokerage, which is mediation between a seller and a buyer to finalize a sale. It is permissible for the broker to take a commission from both parties with their knowledge. As for some customers paying through the bank, if you know that it will involve a forbidden transaction (such as an interest-based loan), then it is not permissible for you to assist them, based on the Almighty's saying: "And do not cooperate in sin and aggression." However, if you are ignorant of their situation, you are not required to ask them about their payment method or the source of their money, and there is no sin upon you if they commit a forbidden act.