I use two Connectors (one for each Lane) when coming from a double-lane Link to a single-lane Link. How can I avoid unneccesary lane change due to the fact that there are two possible routes?

This way of modeling is incorrect. You only need to have one Connector linking the double-lane link with the single-lane link. Because of the route functionality of PTV Vissim, vehicles automatically will change lanes to the one which is connected to the single-lane Link - and thus there is only one possible route.

This kind of modeling is recommended both for static routes and Dynamic Assignment.