With SmartSDR you are limited to what functions you can assign. You can set the current (active) slice volume to be assigned to one of the keys but not for a second slice. The only way is to select the second slice and then use the knob.
If you have an iPad, you might be interested in K6TU Control
which provides a virtual front panel for the radio. This makes touch access to all the controls very fast and doesn't require a mouse action and/or window focus change. You simply select the slice you want to control and then all the controls for that slice are literally at your finger tips.
Email me if you need help - you know where to find me!