I think another contributing factor for Tessio's betrayal, could have been wasn't Tessio more of an 'independent' operator than Clemenza
The novel says that Vito kept Clemenza on a short leash, but treated Tessio with more independence. Tess didn't live at the Mall. Vito tacitly encouraged his foes to think that Tess was an independent operator, the better to hide Vito's full strength from them.
A side question please, what if Clemenza and Tessio teamed up and conspired together
They would not have conspired together because doing so would mean splitting up the Corleone Family. Neither, especially Tess, would have gone for that--it's all or nothing in the Mafia. Also, splitting the Family would gravely weaken both sides.
