My search - I already have a roommate, how do we apply for shared flats (colocations) ?

You can create a single file both of your documents and apply for housing for you and a relative. Remember to inform the different owners that you have a combined file with the message following your application. You do not need many applications; unless it is imperative. During the visit or reservation of your accommodation, you may inform the manager or owner of the housing that you need a separated lease contract for both of you. You can add up to two guarantors.

Remember that not every announce is open to flatsharing. Hence, don't be surprised if you don't get an answer from an announce that wasn't a flat-share.