It is admittedly interesting that despite Mewtwo and Charizard having two Megas, there's a clear distinction in philosophy between them in both cases that is consistent. On the surface one is physical and the other is special, but if you really look at it the Y megas are the "normal" Mega Evolutions of Charizard and Mewtwo in that they are the more traditional Mega Evolution of the two, being a direct stat upgrade in terms of the stat builds of the base Charizard and Mewtwo, and design wise they are much closer to their base forms and feel more like the obvious and natural evolved form of their base forms.
Mega Charizard Y was shown first in marketing iirc alongside Mega Venusaur and Mega Blastoise, and was much closer to what an evolved form of Charizard would look like from the get go and they made it clear it was the "natural" and "normal" Mega Charizard and is much more closely aligned with Mega Venusaur and Mega Blastoise, right down to keeping the same type combo and its statline still being close to the Charmander line's general statline build. Mega Mewtwo Y was also revealed first between the two. Mega Charizard X on the other hand was revealed individually much later as something more comparatively special.
Which brings me to my other point, which is that the X Megas are the more "deviant" Mega Evolutions and feel more like a special Mega and an extreme differentiation from their base form. If the Y Megas, alongside other traditional Megas, are more like the natural evolved form like your typical cross-gen evolution and the natural pinnacle of the line, the X Megas are much more different both in design, type combo, and statline. They feel more like say, Porygon2 to Porygon-Z in that regard, or perhaps Gallade and Froslass compared to the original Ralts and Snorunt lines.
Mega Charizard X has a more radically different statline and its ability and stat boosts focus on a more distinct physical attacker and bulky hit taker build as opposed to Charizard's usual schtick, further compounded by Mega Charizard X being part Dragon-type, ie changing its type combo. Mega Mewtwo X on the other hand is similar, with most of its stat increase going into physical Attack and becoming part Fighting-type, becoming more of a brawn type as opposed to Mewtwo and Mega Mewtwo Y's fast and frail Special Attacker approach, the latter of which is more of a direct upgrade of base Mewtwo a la Mega Charizard Y to Charizard.
Branched Mega Evolutions were still a pretty cool aspect of them in my book and while admittedly it could've been done with more than just Mewtwo and Charizard I do like the distinction in philosophy between the two Megas that prevails in both cases so I wasn't really bothered by them getting two Megas apiece. It's a nice way to add more creativity to how Megas were designed than just sticking to "direct upgrade of the original".
It's a case of where true to the Psychic-type, it relies on mind powers/psionic magic as its power and it shrinks its physical build which ironically allows its psionic power to increase. So while it doesn't look as frightening on the surface, its psychic powers are significantly stronger and it can use them even more effectively. Its LGPE dex entry even alludes to this where its psionic powers are so strong that it can smash a skyscraper to smithereeens with just a mere thought.