assign default sim to a contact/phone number

Hi,
in settings I can specify a default simcard to make calls with (and I did so).
Underneath that option a note states, that it should be possible to assign a certain simcard to individual calls or contacts.
I am fine choosing one or the other simcard for individual calls.

But how can I assign one of my two simcards to certain contacts to make sure I call them from that sim on default?
I would expect to be able to choose a default simcard when editing a contac but could not find anything like that.
Any help would be appreciated.

Aquaris E5, 15.04 OTA 9, German layout