'03 Century Remote Key

When program 2nd remote key for '03 Century, 1st one doesn't work. When reprogram the 1st one, the 2nd one doesn't work. Must be at the 4 limit, but don't know how to remove old remotes of previous owner. How is this done?
Thanks, Bill

I believe the only way to cancel them out is with a scan tool.

They have to be BOTH programmed at the same time......if you do just one, it wipes out the other one....