Yes, you can use one A100 ear bud at a time if you wish to do so. The A100 has a mono mode option which, when enabled, will convert the audio into a mono signal. Mono mode is ideal when sharing music or audio with others as it allows everyone to hear the same thing, rather than there being a separate left and right audio channel.
The A100 will automatically switch to mono mode when only one ear bud is removed from the charging case. You can also enable or disable mono mode by selecting the slider in the app next to ‘Enable mono mode’.