10 stereo pairs for 5 speaker surround, how to do this?

I see what you're trying to do, but I don't know if it's possible. Or even desirable. You may be making it more complicated than necessary.
 
5.1 surround is comprised of 5 mono outputs. Sounds are panned around based on their amplitude relative to the other 3 corner channels (the center channel is its own thing, normally reserved for non-directional low frequencies and dialog).
 
But if you're determined to create 10 stereo pairs, and assuming your audio interface can support them all, create 10 stereo busses, route them to your interface's 10 hardware channels and let the interface's internal mixer handle routing the 20 inputs to the 5 speakers. 
 
When it comes time to export the file, though, it's going to have to be mixed to a standard 5.1 format, as the wave format cannot handle 20 channels (maximum of 6, I believe).
 

But each speaker in a 5.1 setup is fed with a MONO signal, I'm still struggling to imagine why you want or need TEN stereo pairs?

OK, I know there is reticence in doing this because it is UNCONVENTIONAL, but can't you see the advantage of 10 stereo pairs?   Yes my Behringer F1616 has 8 analog discrete outputs, and can handle driving the speakers either 5.1 or 7.1 
 
Multiple channels create 3D space in audio.  A live orchestra has 40-60 or more source points of sound.  Each is in the same acoustic space, but ignore the "natural" space part of this for now.  A pipe organ has a similar acoustic field.  These multidimensional sources give the LIFE to the live music (along with of course good musicianship and teamwork)
 
With a single stereo pair I can create a field that places this multi-dimensional audio in a fairly accurate space for the listener.  I can create the feel of a space using digital effects of course.   HOWEVER accuracy is NOT NEEDED for audio excitement.  In fact, audio used in UNCONVENTIONAL ways is DESIRABLE, to composers and arrangers, who want to create something NEW.  This in fact is what I LIVE for.
 
So now, what do you know about the science of psycho-acouStics?  There a 3 cues for a spatial sound that reaches your ears.  Each of these cues is calculated by your brain to give you a sense of the space of the room and the direction of the primary initial sound.  These cues are intra-aural time differences (the strongest cue) intra-aural amplitude differences, and intra-aural spectral differences.  I can manipulate these cues well with ANY stereo system.  
 
NOW if I have 10 stereo systems, I have LOTS of creative freedom!  You all are thinking like an audio-tech, I'm thinking creatively as a composer.   I can always use some type of surround sound mixer to place the sound inside my audio field ACCURATELY.  Sonar has this, but I want to MANIPULATE the sound by creating space upon space as each stereo pair is defined in a SUBMIX.  My PROBLEMS with doing this right now seems to be I'm LIMITED in how I can assign the OUTPUT to the CHANNEL I want.
 
Right now I can only assign stereo pairs for 1-2, 3-4, 5-6, and 7-8, because of the driver.  I want the OTHER combinations available for creative mixing.
 
What do you guys think?  Do you have any suggestions or workarounds?  I think perhaps the driver setup must be altered, unless there's something I don't know about.
 
Thanks

Clearly my arithmetic is terrible. With eight speakers you get more than 12 pairs. Is it 27?

If you haven't, you may want to look HERE for details/videos on CnB's (Sonar's) Surround Panner (been there since version 4 I believe).  It looks to get very close to what you want to do - per Bit's description above.
 
However, if I were to try your approach, I would try setting up five Busses and/or Aux Tracks (sent to one of the 5 hardware outputs and use channel Sends utilizing a plugin that would allow you to send Left & Right channels of your stereo field separately to whichever bus (speaker) you desire basically giving you designation of which two speakers is your "stereo pair".
 
Whether that's the solution or not, it is the mindset you'll have to follow.  Split the stereo channel and direct those two sides to one of the outputs of your choice.
 
…..but maybe not?