You can place items on content canvas, which will be common for the tab canvases and other items on tab canases.
let say for e.g. you will place the parent items on content canvas with which you are going to populate the tab canases.
Create the content canvas first then drag the tab canvas on to the content canvas from tools palette.
Arrange the tab canvas in such a way that items on the content canvas should not go behind the Tab canvas.
Hope this will help you...