<combobox itemssource="{Binding Combovalues}" selectedvalue="{Binding getvalfromcombo, Mode=OneWayToSource}" height="23" horizontalalignment="Left" margin="64,62,0,0" name="comboBox1" verticalalignment="Top" width="145" />
<textbox text="{Binding getvalfromcombo, Mode=OneWay}" height="23" horizontalalignment="Left" margin="150,134,0,0" name="textBox3" verticalalignment="Top" width="120" />
This way the SelectedValue of the Combo and the Text of the TextBox are pointing to the same Property.
The ComboBox is OneWayToSource in order to just set the property, and the Text is OneWay just to recieve the value of the property, not modify it.
If you want that, if you edit the value of the text, the combobox should also change, you must set the Mode=TwoWay in both controls or just delete it, because it is TwoWay by default.