Since I was asked to make a response here, I shall.
So, some points to be made. These are based on the original lore and as such it's a take it or leave it.
Cognatism is the act of convincing a portion of your mind that an illusion is real, able to switch between this portion which is labeled as the "Spectator mind", switching puts you into a trance-like state. However, some flaws exist with this.
First off, the act of convincing yourself an illusion is real requires a certain personality and mindset, it's certainly not for everyone. Not just this but... imagine a Cognat's mind after this process: traversing through it would be... pretty difficult. The emotions and feelings a mental mage would read from a Cognat's mind would be very jarring and sporadic to say the least. Memories would be hard to draw forward due to this finite balance a cognat has achieved with this process - by default Cognatism is a natural barrier against mental magic due to the fact that just entering would be like seeing two doors which lead into a house which is only half complete, and only one door can be opened at a time. Memories would be scattered between the minds, and entering a dormant spectator mind... is something a mage should avoid for their mental health.
With this in mind the question comes up - can mental magic be learned if you have cognatism or vice versa? The old guide to Cognatism said not only yes but that it was one of the requirements. This... is a little bit incorrect but it's a bit too late to change things, I suppose. So, let's put this in perspective: Cognatism is the act of building a way for yourself to view illusions as real in a trance-like state. Mental magic is the act of finding a mind through the void and communicating with it through words, pictures, or emotions. By default one would assume that mental magic requires a great deal of mental stability from the acting user, how can you find any mind in the void if you can't even touch or access your spectator mind in the slightest unless you switch entirely? The fear of possibly puncturing or accidentally drawing memories or thoughts from that sector of your mind would probably put you off knowing full well that doing so would likely result in very quick insanity when you manage to get an infinite feed loop.
This isn't even mentioning the fact as well that you would likely need to maintain active control over your mind at all stages to keep the Spectator mind in it's dormant state as it's not like it's some box you open only when you wish to - split personalities hardly are that simple. Personally, I would probably say that mental magic and Cognatism are in-compatible - even to the portion to say that Cognats are extremely resistant to mental magic. Under this logic, merging them would be... silly, to say the least.
However! I acknowledge, Cognatism is not that useful of a magic for those who wish it less for the RP value and more for the character power factor. Under this, I feel like Cognatism shouldn't take a magic slot due to being the basic equivalent to mage house magic. It's "neat"! It's not practical. I don't feel like merging them is the answer however.