This recently came up and I am not sure how to handle this. My group has a Rune Priest that wants to sustain multiple instances of Living Lightning. Now, my first reaction was to say that this would break the system, as having two Psy-rating 2 LLs seems better than having one Psy-rating 3 LL (at least when fighting Hordes), but then I realized that on the other hand it seriously reduces the ability to harm (and shock) Elite and Master enemies. Not to mention the chance of worse Psychic Phenomena as normal for sustaining multiple powers.
The only thing I could find on this topic, is a note in the errata saying that a Rune Priest can only sustain one instance of Tempest's Wrath. Seeing that here it seems to explicitly make an exception, this leads me to believe that normally you can sustain multiple instances of the same power.
What is your take on this, my dear forumites?