AWS保留实例多个EC2同时运行

If I have 2 equal EC2 instances running simultaneously for 12 h a day and I have only 1 reserved instance, does this reserved instance cover the cost of both instances since the sum of daily hours is 24h? Or however, since the 2 works simultaneously, would not the reserved one cover me and would they apply extra expenses? Thanks in advance. If I have 2 equal EC2 instances running simulta