What? You're just going to assume they fought despite nothing stating such (nor remotely implying either)?
Revan's Sith Empire didn't function like your classic master-apprentice duos, obviously. They didn't train apprentices from birth and a lot of their mastery with the blade came from experience in the battlefield. It's said that when Darth Bandon became Shadow Hand, he wasted "no time" and began killing his enemies. That makes sense, since Darth Malak picked him due to his excellent combat skills and power in the Sith Academy. Thus, it's apparent Darth Bandon's combat skills were already existent when he got the title - Malak likely just taught him Sith secrets.