Mixes 1-2 3-4 can not be linked however you can switch the groups to function as stereo mixes.
That said on the SB there are no group outputs so you would need to have a stage box connected to the mixer like a AB168 to have more outputs.
You can assign on of the group/mix outs to the ALT OUT.
I use fruityloops and pan input 1 left and pan input 2 right.
To use stereo FX I route them to a free stereo software mixer channel.
I do the same for input 3 and 4.