When do you need to copy a broken key?
Depending on your technical position, this may not be the problem you are looking for, "you can copy the broken key." Although it is still a sure answer. Even if your key is broken, you can get a copy, but you may not copy the broken key in the strictest sense.
You don't necessarily need to have both parts at the same time to copy the damaged key. However, you need something that can be used to gather information from it. The most important part of the broken key is the full grooved blade. If this is the one you have and it has been locked on the shoulder of the key, then you may only have to copy a broken key.