I too would like the option of syncing a single SD across multiple devices. However in another thread, gustavwiz suggested a more flexible option:
"That's because many want to separate their home and work speed dials. At the same time, many also want to have the same speed dial at some places. For example work speed dial on work computer, and home speed dial on home laptop and home desktop. That's why we want Opera to let the user be able to choose."
So a toggle/checkbox etc that lets a user switch between 'same SD on all devices' and 'unique SD on all devices' would not be sufficient for some.
I would love to see gustavwiz's suggestion implemented.