We also have one PTO that supports 2 elementary schools. The problem we have is that one school's representatives are controlling and shut down ideas from the other school's representatives. Their attitude is "we've always done it this way"and are reluctant to change. How does your PTO work? Do you split funds 50/50? Does your PTO have the same assemblies and activities for both schools?
You need to do some research and see if you can find by-laws and/or articles of incorporation -- eg, if your group is officially titled "X and Y elementary PTO" then it'll be hard to add the middle school.
Our group was formed years ago. All the original members have long gone. Most of the members are fairly new to the rules and bylaws. We don't know how to change things. We don't even know where to look.
Currently our group supports 2 elementary schools. Our new principal is now also over the Middle school. He asked if we could look into supporting the middle school. No one has a problem with it but we would like to research things. Main question that is being debated and holding up things is - Can we add them to our group and How do we add them to our group?