I have a few enclosures with 2 power supplies. I have both connected at -12. One power supply feeds the controller. The other is used for power injection. I was also told that I would see "dueling", but when I pushed back I was told that it would work. Well, I have been using this method for a while and so far haven't had any power supply issues or pixel problems.
Here is info on power injection.
http://www.doityourselfchristmas.com/wiki/index.php?title=Power_InjectionAt the bottom is a photo showing an acceptable method for two power supplies. I attached the picture also (this is basically how my setup is, except I only connect my power injection at the ends). If you look, one power supply feeds the controller which powers the first string. The second power supply is feeding the 2nd string. However, in reality, the 1st power supply is feeding the 2nd string as well and the 2nd power supply is feeding the 1st string. So this would be "dueling". But, it just isn't.
One thing to note, I have better luck using voltage around 11.8-11.9. I noticed if I have 12.0, the chips seemed sensitive enough to where they would "glow" red on occasions (not always). This would happen with only one power supply (before someone says that's a symptom of dueling). Once I lowered the voltage this glow didn't happen anymore.
James