You can. Each engine can be assigned separate RPM/Prop pitch controls. I sometimes pull back RPM on one engine to save fuel while cruising in situations where I don't want to lose too much speed. It's not a huge difference from just pulling both throttles back so I don't do it often to be honest.
This. In constant-speed props, RPM controls feathering. Cut RPMs all the way back to feather. My X55 has both throttles and both RPMs mapped to quickly feather one.
