You're using gameplay mechanics over story mechanics, in which a Double Rock On is stronger than EITHER of the two Biometals alone.
Nonononono, I'm not.
I'm counting the doubles as part of Model X's power just as much as I'm counting the Trans On forms as part of Model A's power.
Discounting those transformations, it is only natural that you get Model AX > Model X > Model A > Trans On X from a single form perspective.
But from a multi form perspective.
Model X has the most potential power as it can bring out the full potential of all livemetals, including AX. AX can copy all singulars at their regular power. A can copy doubles and singulars at less power. A can copy X, which can double Rock On, but that results in exactly the same as A copying a double in the first place. And you know Model A is stronger than any of its Trans forms by definition.
Therefore Model X > Model AX > Model A > Trans Model X
Just look at the forms each has at their theoretical disposal:
Model X: full potential ZX, HX, LX, PX, FX, VX, OX, AX. Regular Z, H, L, P, F, V, O, bosses.
Model AX: regular Z, H, L, P, F, V, O, bosses.
Model A: weakened Z, H, L, P, F, V, O, X. Weakened ZX, HX, LX, PX, FX, VX, OX.
Weakened X: Weakened ZX, HX, LX, PX, FX, VX, OX.
Actually, I take that back, Model X has the most variety. It can literally become anything that A can also be, but at greater power.
From a game balance perspective, I can almost imagine AX actually lacking a trans on in favor of stronger homing and giga attack. But, just as easily I can expect a Trans On X form to lack the double Rock On. Simplifying the list to:
Model X: full potential ZX, HX, LX, PX, FX, VX, OX, AX.
Model A: weakened Z, H, L, P, F, V, O, X. Weakened ZX, HX, LX, PX, FX, VX, OX. bosses.
Of course, the main conclusion doesn't change. Once you have the Model A as Rockman Model X, you're pretty much the ultimate Rockman to begin with. AX's possible lack of Trans and the possible inability to use the other singulars are just a couple of points that change how exactly you can use this power.