Cristina,
With object dependency assignment to the Routing operation during Sales order entry with selected standard value characteristic it will overwrite the values only which is standard behavior of the system.
I am not sure about your business requirement why you want to multiply the Routing standard value with Sales order selected characteristic which could not be possible with system standard design.
If you want to include both Routing operation standard values and Sales order selected characteristic values during scheduling you can have 2 different operations one with Object dependency assignment and another with out any object dependency assignment so that 2 values are included in order scheduling.
BR
KK