This is a tricky situation for any employee but I would advise against hanging up on the person you are already on the phone with - you can always ring the second person back when you have finished.
I was in the hotel industry for a couple of years and we were always told to be polite at all times. I think it in this situation it would be important to be patient and wait for the customer to finish deciding what they want to order.
Once you have finished the conversation with the first customer and are confident that they have ordered everything they want, you can then ring the second customer back.
When you ring them back it may be advisable to apologise for the delay and any inconvenience caused. This way both customers would feel that they are being taken care of.
If I was a customer ordering room service I would not like to be rushed into picking something, or have the phone put down on me.
I wouldn't find it so bad however, to be ringing for room service and to find the phone engaged. I would either ring back later or would wait for the hotel to ring me back once they have finished taking the call that they are currently on.
