We are taught in Aqeedah that the ash'aris deny many of the hadiths that contradict their creed by saying they are ahaad, and that the correct way is to accept ahaad narrations even in aqaa'id. Examples are given of the ambassadors to various nations being sent alone, and Mu'ad ibn Jabal being sent to Yemen alone etc. But in hadeeth lectures it’s said that Abu Bakr used to not take one sahabi as sufficient and required a witness in order to accept a hadith. Also my personal addition is that I have also heard that Umar(RA) alsodid the same when he was told the hadith about knocking thrice.How do we reconcile this?

What Abu Bakr and Umar (peace be upon them) were doing is just putting forward a mechanism to preserve hadeeth, making sure that people do not narrate something that they are not sure of or narrate something based on doubt and forgetfulness. This does not in any way mean that a narration of one Sahabi is not acceptable, because Aisha (peace be upon her) narrated hadeeths of events that happened between her and the Prophet (ﷺ) in their house and these are universally acceptable hadeeths.