We can allow the customers to create order for single brand. If they want to create order for including the multiple brands we should restrict in cart page. cart items with same brand allow for sales order,
You should create a new attribute, for example: "Product Brand", and then add custom code to review if a product from another brand already exist in the cart.
That can be done in an observer, and prevent adding products to the cart if there is another brand in the cart.
Please provide the code as urgent basis.