We are a PTO that serves two buildings, one per-k through 3, the other, grades 4 & 5. I have tried for years to have separate parent groups for these schools BUT have come to the realization that the older group probably would not be able to sustain itself.
We have several families with students in two, OR MORE, buildings, my sister is one of them with 5th, 7th and 10th graders in three buildings. There are many ways to "spread the volunteer wealth". I guess it's really a matter of deciding what works best for you and your family and how much time and/or energy you are able to share. Some of our families help out more with the younger kids, simply because there are more volunteer opportunities at that level.

Yours is not a unique situation. Last year I had a child in elementary and a child in middle school. Next year I'll have one in middle and one in high school. For parents who want to be active in their children's school it is a balancing act. I made the effort to involve myself as evenly as I could at both schools and supported both schools in the same amount financially, and I will continue that effort. Since the two schools are both elementary, I wouldn't think it would be that hard to team up for some of the events like the reading nights. Maybe some of the parents should get together and find areas where the two schools can do things in conjunction.