ARTCC Supervisor described an inappropriate combining of position/s event when controllers failed to comply with appropriate directives.

Narrative

Holding traffic in two sectors. After holding completed; both sector controllers asked to combine sectors. After analysis of combined sector traffic; I authorized the combination of the sectors. Approximately 20 minutes later; I overheard the combined sector controller remark to his D-Side that '...he'll figure it out sometime.' I analyzed the area situation and noticed that the sector that had been authorized to be combined was still an active sector in HOST. I immediately rectified this situation by staffing the open sector. Both involved controllers did not advise anyone that the sector authorized to be combined was still open. FAA Orders clearly state that it is the responsibility of the combining controller not to abandon a live position until such position is officially closed. The fact that the combined sector controller noticed this and made a conscious decision to not advise me or take any action whatsoever is reprehensible. The responsibility for combining/decombining sectors is mine; yet there the system demands that anyone observing an unsafe situation immediately report it before something happens. Recommendation; education of the workforce of responsibilities for combing/decombining sectors and all applicable orders. Education of the workforce on the importance of immediately reporting unsafe situations.