Second LED bulb in each fixture burns out, what should I replace?
Older dimmer switches are not necessarily compatible with "dimmable" LED lamps. Replace the dimmers with those designed for LED's, or replace with a standard wall switch, and use remotely-controlled lamps.

BTW, there is no problem disposing of "dead" LED lamps -- most should comply with RoHS and contain little or no hazardous material. While the emitters may contain tiny amounts of arsenides, they're well sealed in plastic. Perhaps you're confusing LED's with fluorescent lamps, which have small amounts of metallic mercury.