Is it permissible for a broker to take a commission from both parties without their knowledge, such as agreeing with one party on a 50% profit/loss ratio and offering the other party a 40% ratio to achieve 10% for himself without their knowledge?

If the transaction is a brokerage, then the broker must inform them of his commission. If it is a Mudarabah (profit-sharing) and they have authorized him to give their funds to another Mudarib, then he is permitted to take the agreed-upon commission (40%) without their knowledge. The knowledge of the aforementioned person is not required in either case.