Hi Deborah!
What;s something you always agree on? [you don't need both if speaking to the people in reality]
What’s the one thing you both always share the same OPINION ON?
What's the one thing you both always agree on? This is good - it also could be 'What's something you both always agree on?' or 'Can you tell me about something you both always agree on?'
Cheers, Tutor Zowee :)