What is the tenant's share of the water bill, and does the contract obligate him to pay 60% after he stated that he is not using the additional apartment?

In the case of consumers sharing a single meter, there is no impediment to agreeing upon certain percentages for paying bills, due to the difficulty of determining each individual's consumption. However, this agreement is not binding, and any party has the right to withdraw from it if they deem it unfair. Consequently, the tenant is not obliged to pay any percentage or amount as long as they are not consuming water, because payment is in exchange for consumption.