Fletcher has been a regular source of goals for the Black Cats since being snapped up in a big-money transfer from Wolves.
Graham was drafted in from Swansea during the January window to provide support, but has been used as a substitute so far.
It has been suggested that O'Neill is reluctant to tinker with a lone frontman approach, despite bolstering his ranks.
He insists that is not the case, with it his intention to unleash a two-pronged attack at some stage.
O'Neill said: "I don't think we have got one specific system - the two players can play together.
"I don't see that being a massive problem, but it is bound to take a little time."