I am not familiar with Superior Drummer, but that is where you need to make a change. Once Superior Drummer is configured for more outputs thay will show up in the VST drop down menu, where you will be able to enable them in Cubase.
I found the following on page 37 of the Superior Drummer 2 manual:
By default SUPERIOR outputs into your host as any stereo instrument you may be familiar with> .
It can, however, be addressed as a multichannel instrument in most compatible hosts so that further
processing can be applied, allowing you to use your entire arsenal of effects from within your sequencer.
Clicking the ‘Output’ field at the bottom of the channel strip (in orange letters, over black), will reveal the
possible outputs for this particular channel. Microphone channels can be routed to any bus or output.
You can also disable the post fader signal and rely solely on the bus send sections if you wish.
As I said. I’m not familiar with Superior Drummer so I don’t really know. On the surface it sounds like you have mono channels in the Superior Drummer Mixer instead of stereo channels.
Page 54 of the manual:
The Settings window contains the sampler preferences.
• Mono All Channels - outputs are treated as independent mono channels. This is useful for some hosts or
for creative routing, giving you access to 32 individual channels. This configuration applies globally to all your
projects so do not select if you are in the middle of a project, working to a tight deadline!