Thanks, you all are quick! i did some more searching here and found answers in other threads relevant to my question, which i should have done first. apologies. In other forums i try not to start my own threads when there are others with the info i’m seeking or hijack someone else’s (broke my own rule 
You all are so polite here from what i’ve seen so far!
But it is not quite accurate to add them together if the schools give very different FA amount. Say your EFC is #20k for each kid so you expect to pay $40k (which isn’t correct but just go with that it is). If one school gives aid so that your really do only pay $20k for that student, the other school may expect you to pay $60k. You don’t get to say “Hey, I’m already paying $20k to state U. I can only pay you $20k and that would be our $40k.”
Some schools just don’t have the aid to give you up to the $20k or even $40k. The cost is the cost. Two schools that have a posted COA of the same amount can end up being very different if one gives aid and the other doesn’t.
One of my kids had more in savings account than the other, that caused a $400 difference in their FAFSA EFCs.
Income can also be different between siblings, if it was much higher than $7,000 or so, it can contribute to EFC.
Basically our FAFSA EFC number meant that we qualified for state grant but not Pell Grant.
The school based the need calculation on that EFC but didn’t give aid other than state grant, federal subsidized loan and work study.