Using multiple frame in a wpf window

i'm developing an application using wpf by c#. i have 2 frame next to each other. the above will not change and just updated(like facebook toolbar). but the bottom frame will change by clicking on some buttons. (it is something like masterpage in ASP). i don't want to set a height and width for window or for frames. the problem is when i make the window full screen there is a space between two frames. however non of the vertical/horizontal alignment stretch does not work. can u help me?