I have no medical background but I had terrible heel pain and found those heel inserts stopped my pain in a few days. I wouldn't have thought you could use more than one at a time and I'd imagine that, if you tried, they'd overlap.
you can only use one. my favorite over the counter insert is "superfeet". they are sold at sporting good stores like sports authority, REI/EMS.
doing calf stretches help too. if they continue to bother you, get a coritsone injection.
from my experience, heel cups don't do the job well.