I created a skill with a few intents, one of which had two custom slots. I pasted in the intent file, the utterances file, and added one slot. At that point the UI complained saying the second slot was missing and wouldn't let me save. But, because it wouldn't let me save the first slot, the controls were greyed out. If I entered the second slot first, the same thing happened, complaining about the first slot. >_< As a workaround, I had to delete the intent with the custom slot from the intents and utterances files. Save it. Add each slot. Save it. Then add back in the full intents and utterances file.