You can put the textboxes inside a grid to do percentage values on the rows or columns of the grid and let the textboxes auto-fill to their parent cells (as they will by default). Example:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*" />
<ColumnDefinition Width="3*" />
</Grid.ColumnDefinitions>
<TextBox Grid.Column="0" />
<TextBox Grid.Column="1" />
</Grid>
This will make #1 2/5 of the width, and #2 3/5.