Ah yes, the CSS - the colonoscopy of your finances ;-)... Be careful to assume each school takes some percentage off, but it is usually close to half for two. I can also say it got better when #3 overlapped because he went to a school with a $20+K less in tuition/fees (hard to believe it was almost 10 years ago).
When my first two boys overlapped at two different D3 HA schools, I got a fairly crappy "offer" from one school. I did a bit a research and found on their website an article from my others son's school as a guide for how aid "should be done". Since I of course had numbers from the other school I pointed it out to the FA office. Essentially the article noted that research the school had done was finding kids were leaving with higher and unmanageable debt, so to help they had altered their formula removing money from other budgets such as new buildings. When I pointed all this out to the first school of course they weren't happy, the officer even said something to the effect of this is why they rarely "engage with parents" over the matter. In the long run, they gave me like $1k more which really didn't help all that much, but when you're a year or so in it's kind of hard to go back to square 1. FWIW, my response to him about engaging with us was to remind him to take us and our son off any future give back to your school campaigns. What comes around goes around, if you're as smart as you think you are you'll actually read the advice from the first school.