For members of the same household, one contract is usually sufficient. In family insurance, all unmarried children are generally co-insured if they are still in education or military/civil service.
If a couple moves in together, both can be insured under one contract. The longest-standing contract is continued, and the "younger" policy can be cancelled without notice. However, there is no liability between joint contracts. So, if the new housemate damages the expensive stereo system, the insurance will not cover it. Such double insurance should be avoided.