(Author's note. I'm ignoring the "change reality for everyone" part--because everyone else is.)
Karyn decided. She held the stone and said "I wish that I was in Jon's room right now, and that Jon knows this is a wish."
Karyn materialized in Jon's room. The wish came true for Jon too, of course, but Jon was already there.
"Karyn!" exclaimed Jon as his friend appeared out of thin air. "So you really do... but wait. That can't be right. Anyway I was getting ready to go back to sleep."
"I wish that Jon is fully rested as if he's had a night's sleep without being interrupted," said Karyn. "Feel any better? Something's strange here. I'm a little afraid of what could have happened... magic going bad could be anything."
"I don't know," said Jon. He didn't feel quite as tired... "I guess it won't hurt if we talked it over now. But I know a shortcut. I wish I knew why Karyn and I both have a wishing stone."
Karyn gasped. The wish affected her, so she knew what was going on too! "The stones... they're not the same one..."
"I know now," said Jon. "And since the stone affected you, so do you."
"I can't believe what we could have done," said Karyn. "Suppose I didn't talk to you and I used the stone to get dressed or something like that?"
Jon nodded. "And the real stone... we need to get it back. But we've got a problem. Suppose either of us needs the stone before we can get the real one back?"
"Well," said Karyn, "We've lived without a magic stone for 17 years. We can do it for a little longer."
"Yeah, but still. There could be an emergency. Besides, if we can't use magic, we probably will never find the real stone. Let me see... I can't just take both stones myself, because the two stones won't accept the same owner... the second stone will still think it's owned by you even if it's a mile away. I think we have only one choice, Karyn."
"What did you have in mind?"
"We each keep one of the twin stones. When we need to make a wish, we just have to make one that won't hurt the other person. Like when you made me rested--it's okay for both of us to be rested. If you have a cold you can wish to be healthy, since it'll just make me healthy too." Carefully putting down his stone, Jon continued, "Just as long as you don't say 'I wish I was healthy so my boyfriend won't catch anything from me' or something stupid like that."
Karyn nodded. "Seems like a good plan for now."
Later that morning they went to school, each of them carrying one of the twin stones. But could they really avoid making even one mistake with them?
