Lenz Digital plus LH101 Mode D'emploi page 169

Double traction
Press the '<=' key to return to "Controlling locomotive".
One of the two locomotives that you wanted to add to the
double reaction has already been called up by another
manual control.
One of the two locomotives that you wanted to add to the
double traction is already part of another double traction.
The speed step of one of the two locomotives was not 0.
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