Depends on when you want to synchronize the plot:

If you want to have them synced, while the panner is active the only "easy" solution I see is to send mouse events ( or implement a bridge calling the protected widgetXYEvent handler ) to panners of the plots - remote controlling them.

If it is good enough to sync the scales, after panning has been done, you can simply connect to the QwtScaleWidget::scaleDivChanged() signal. Note, that is a common way how to sync plot scales according to user interaction - not only for the panner. But avoid creating an endless signal ping pong, f.e by disabling the connections temporarily inside of the slot.

Uwe