OK,
So I decided to install and play Frozen Bubble which, oddly enough, appeared on both screens … “No matter,” Thought I. “It’s happened on Windows before and it recovers when I exit.”
Only it didn’t. I came out of the game to find I had a single screen desktop still duplicated across both screens … how do I reset it?
Thx 
Keke
What DE? I’ve had that happen with KDE on 42.1.
Try Configure Desktop -> Display & Monitor and drag one monitor from on top of the other.
Yes, KDE Plasma 42.2 … I thought that was what the “LEAP 42.2” I selected as my OS meant? Sorry but I’m new to this.
I could get to “Configure Desktop” by right-clicking the desktop but I couldn’t find what you suggest there. I actually found it eventually in System –> System Settings –> Display & Monitor so thanks for that 
Keke