Grid.ColumnSpan animation

I want to archive the following behaviour of my controls:

I got 2 Controls. the one on the left side contains a dataGrid with 1 column, the one on the right side a bunch of other controls.
When i now klick a part of the left one (the green arrow) i want it to expand over the right control and add some columns to my datagrid.
Problem is: i dont know the width of the right component as it fits to the window-width.
Attachment 30377

Attachment 30379

anyone got an idea how i can archive this?