No worry, a 2 bed room 公寓 is enough for 4 people 同住 (我们夫妻俩+父母). Even for a year. You can ask landlord to confirm. I think 合同只是说如果没有登记的人与我们同住我们必须通知他们 is for security or say safe reasons only. Or, some landlord may ask you pay some for more people use utilities, that's could not be too much. 合同 should has this conditions.