I assume you can't sink and source with the same SMU at the same time?
No, it can only source/sink either current or voltage (but not both) at a time per output. There is a model or two that have more than one output though.
Also, whatever your sourcing you set a maximum limit for the "measure" function. IE, when sourcing Voltage, you set a current limit. When sourcing Current, you set a voltage limit.
Edit: Technically, you can do strange things like connecting two SMUs, and have SMU#1 sink current, and measure voltage. Then have a SMU#2 sink voltage and measure current. Then SMU#1 would show both negative current and negative voltage on its display. Although still, there is only a single source for each unit, regardless if it is positive or negative.