Yeah, the way orders are typically (if I can use a technical term) "pipelined", it would probably mess up future orders or maybe lost orders.
If a second order from the same vehicle would mess up or "lose" orders, that restaurant has some serious problems.
Car pulls up to the speaker:
Worker: How can I help you?
Driver: I have two orders.
Worker: Go ahead with order 1
Driver: Places order
Worker. That will be $xx.xx. Go ahead with order 2
Driver: Places order.
Worker: That will be $yy.yy, please pull forward.
Car pull to window.
Worker: That will be $xx.xx, (takes payment) please pull up to get your food.
Driver: We have the next order also (can provide what's ordered if need be)
Worker. Oh, ok. That will be $yy.yy. (takes payment) please pull up to get your food.
Car pulls to food window
Worker: Here's your order (order #1)
Driver: We also had (fill in the blank)
Worker: Oh, OK, here's that order (order #2).
Having two orders from the same car should be treated like two orders in two different cars.