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.
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.