You need to first assign SmartShades to a specific Group before you can control them simultaneously. See here for instructions.
Once you've successfully grouped together shades, follow the steps below to move all of the shades assigned to a group at the same time.
Tap Groups from the main dashboard.
Select the Group of shades that you would like to move.
The control screen features a slider, which is a representation of all of the window shades in the group. Move the notch (circled in red) to the precise position you would like all of the shades within the group to move to.
Note that although each shade within the group might begin at different starting positions, they all will move to the same relative position.