I know from my glider flying that ANY control surface movement sucks up energy from the model's motion. From this I'd say that it's the roll rate that eat's up the energy rather than deflecting one or two surfaces. So I would say it's best to roll into a turn slowly using as small an amount of throw as you can live with. But then one surface has less hinge line length.....

It's all a compromise, isn't it........

But I would think you'd be more at risk of stalling or causing draggy flow separation using one surface deflected farther than two surfaces deflected less. And a fairly clean and well sealed hinge line has very little drag.