From what I remember, during the scene where Akka teaches Kellhus, he explains that the Anagosis and Gnosis use 1 utteral string and 1 inutteral to better fix the meanings.
When Kellhus asks about adding a second inutteral Akka mentions this as the famed "Third Phase". Which would suggest that it's something kind of possible and not just a one-cant thing or a single incident.
Still, it's true that so far we know only one cant which actually uses a second inutteral! But that could very well be only because we see it in the last Kellhus POV.
Maybe in a future POV we'll see other MG Cants where it's specified that they all use 2 inutterals.
Also regarding the use of 2 inutterals in regular cants, from what I understand the second inutteral added by Kellhus helps him to obtain purer meanings. We know that in sorcery purer meanings usually equals stronger Cants so I recon Kellhus can theoretically turn any regular cant into a meta-gnostic one by adding a second inutteral or something close to this.
Just to clarify, Akka actually tells Kellhus that it's impossible to use a a 2nd inutteral. He then recalls, in his mind, a Mandate/Quya fable that there once was a schoolman/quya (can't remember which) who was said to use 2 inutterals. Though it doesn't really make much difference, since we know that Kellhus would have just "read" the truth from his face anyway.
If the Gnosis is some kind of logic based geometric sorcery:
Consider that within a 3 dimensional plane , you can only define a point with 1 coordinate. i.e (x). This is a 1d "object".
I think this is, more or less, what the anagogic schools use. They are limited to defining 1d sorcerery. If they want to "make" something, they can only specify 1 thing at a time. For example, something's length, or somethings height, but not both. This makes their "meaning"(purity) unclear.
With 2 points, you may define 2 points. (x, y). With this, you can now make your meaning much more clear. You can define something's length and width, or its height and length.
This is where the Gnosis is. The Mandati, and others, use the innuteral to clarify what they want more accuratly.
With an additional innuteral (the Meta-Gnosis) you can more fully describe what you want.
As a crude example: An anagogic schoolman, a gnostic schoolman, and a meta-gnostic schoolman want to summon a red cube that is 10meters long per side.
The anagogic schoolman simply says: Summon cube! He gets a transparent cube thats 1 meter long.
The Gnostic schoolman says: Summon cube, but thinks in his mind Red. He gets a red cube thats 20 meters long.
The meta-gnostic schoolman says: Summon cube, thinks Red and 10meters. He gets a red cube that is 10 meters long.
Sorry for the extraordinarily lame example, but its easier for me to think about than simply "and innuteral makes it more pure, and a 2nd makes it more purer ".
I feel like I'm mostly just rambling now so I'll shutup.