You can have only one description.ext. You can put into it as many dialogues, and as many everthing elses, as you like. Obviously you need to ensure that you have different names for everything. The game usually crashes very quickly and quietly when you make a mistake in description.ext, but the error message you get after the crash can be very helpful.
Remember that you have to save your mission after editing description.ext for the changes to take effect.
There is a detailed tute on dialogues, I think it is by vektorboson but I'm not sure. Won't help specifically with the dialogue maker, but it will help you understand the background.