The term tethering or double docking refers to how river cruise ships are sometimes docked. In Europe, all river cruise ships share a dock. This means that your ship is very likely to be docked between other river ships.
Please keep in mind that whilst your ship is docked alongside one or two other vessels, the view from your cabin may be obstructed.
It is also very important to consider that during this time, you will have to embark/disembark your ship by walking through the sun deck or the lobby of the adjacent ship to reach the shore.