I've been playing Ultra Moon since December and I have never been able to use two Z-Moves. What are the requirements?
You need to max out your relationship/affection with Rotomdex to use Z-move twice in a battle. If you max out, there will be a higher chance for Rotom Dex to let you use Z moves twice when one of your Pokemon faint.
>>While on your journey, you’ll notice Rotom on the bottom screen of your 3DS. There will be certain times where he looks sad. This means he is lonely and wants some attention or talk. Simply tap the side of Rotom’s face with your stylus to make it happy. Doing this will either make it happy or prompt a question. Answer the question to build your relationship with your Rotomdex.