Separate green roofs, separate systems

Hi, If there are two areas in the building that will be irrigated by two separate systems, how can this be taken into acount in the LEED template to calculate if 100% potable water is being saved? If I input the rain water expected to be stored and used for irrigation the template says I could go for 4 credits but since this rainwater is only used in one of the green roofs, I would technically be "cheating". I could go ahead and make the calculations myself but it just seems like the template should allow for this. Just wodering if there is a way to take this into account in the template. Thanks,