Feed the 10'' bass in till it's full. Then add the 6'' while doing a 50% water change after you feed the bass. Worked for me with a 14'' bass and a 6''er.
Its risky. Maybe not from the point of predation, but on bullying. If thats an alpha fish he will beat the smaller fish to a pulp in short order. In a tank that size theyve got no where to run.