Sharing layers between canvases
You can share a layer between multiple canvases. That way, whenever you change the layer, it affects every canvas that shares that layer.
To share a layer between canvases:
Choose New Shared Layer from the Action pop-up menu at the bottom of the canvases sidebar.
The shared layer appears in a distinctive color and with a shared-layer icon to the left, to remind you of its special nature. The exact color depends on your highlight color as set in the Appearance pane of Mac OS X System Preferences; for example if you use the default blue highlight color, your shared layers are orange.
Add and edit objects on the layer normally.
Whenever you want to start sharing the layer, just drag it to any other canvas in the sidebar. Another copy of the shared layer appears inside that canvas. You can reorder the shared layers freely.
Any further changes you make to any copy of the layer are also applied to all other copies of it.
Objects on shared layers, the corresponding items in the list sidebar, and shared layer previews in the canvases sidebar all appear in the distinctive shared layer color, to remind you that you could be affecting many canvases if you edit them. 
You can remove a copy of a shared layer from a canvas by deleting it normally. If you delete the last copy of a shared layer, it is deleted completely. There is also a Delete All Copies command in the Aciton pop-up menu at the bottom of the canvases sidebar.