unique between 2 columns

i need to create a validation to my table "candidate_knowledges", basically in this table it accepts to columns (candidate_id, software_id), i cannot let create user_id and software more then one. but i think my validation is wrong or im not doing it right. What im trying to say in validation is that can only exist one software_id and one candidate_id on the table, this way the candidate dont have duplicate entries.

Ex:  'software_id'         => 'required|integer|unique:candidate_knowledges,candidate_id,'.$candidate->id,

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