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?

No.  How would you cast the second one? It takes a full action to use and you have to use a half action to sustain the first one.

It is a full action to manifest so there is no possibility of sustaining 2 of them, as Nathiel stated.

Even if it wasn't a full action, I still would not allow it. The power does not state the lightning strikes are automatic calling down the bolt (as part of the half action to sustain) should still count as an attack action. This does not overide or allow the "one attack action a turn" rule to be ignored. You can still sustain the power and use other attacks if you don't call it down for that turn, but you can not attack and call a bolt down (another attack), and you deffinately can not attack, call a bolt down, then call another bolt down (3 attack actions).