That stereo effect is more or less always present, as the OSCs are out of phase. There is also a prameter called "detune" that widens the stereo image.
When in OSC menu, you can hold shift and then select left/right/both SID(s) with the first button below the display. Then you can control each SIDs settings individually.
Different midi channel per OSC can be done via Ensemble->INS.