MegaMerge speculation.

And keep in mind, this is speculation. So treat it as hypothetical. And is just another alternative theory.

Taking into the account of the Double MegaMerge and the Solo MegaMerge, there are a few differences between the 2.
Model X has been shown to be the only Model capable of Double MegaMerging.

This could be for at least a few reasons:

-A: Model X being based off of X who was, in a way, the father of most reploids.
-B: The 4 Guardian Models are based from the Big 4, who were variants of X's DNA.
-C: X could have been the first successful Model derived from Model W, and the could have been designed off the bare specs to create the others.
-D: X being destroyed sealing the Dark Elf, when X was the only reploid to utilize Mother Elf to end the Maverick Wars. X cyber self could have adapted to Mother Elf, or has a Link with Mother Elf enabling him to unify with Reploids.

Unification with Model Z could also be a few reasons:

-A: Zero being transferred into a Copied Reploid Body, could have been based off of the ultimate specs of X. As the other reploid majority.
-B: X and Zero, being fully analyzed late within the Maverick War, there could be a design similarity. (Wily could have used the core blueprints to design Zero. Or vice versa.)
-C: Zero was recognized by Dark Elf. Suggesting some connection with Mother Elf in either way of the Virus, or other means.

Of course that is merely speculation, but either option currently present does assist explanations.

Following from Model X, it is possible for there to be Model Ax. But with the connection between X and Zero being limited, it is improbable for there to be a Model Az. And certainly improbable for anything to be fused with Model A. With it's A-Trans being the Model Prime function could have complications.

According to the other Models, when Fused with Model X, Characteristics increase in both power output and technical alteration, plus Overdrive. When stand alone, the Models are less bulky, no Overdrive and overall look more independent.
This is proven in both ZX and ZX:A.

Now the only difference here, between Model X and Model Z, is the link to Model O. Now so far, Model O is nothing more than a refined BioStone possessing the affinity of BioMetal. X is able to fuse with Model O with X's Double MegaMerge. However, Model Z would have a connection with Model O in that they both derive from Zero. Model Z being based off of Zero's consciousness transferred into the reploid copy body, and Model O being based off of Zero's original Body. Omega in a sense being Maverick Zero.

With that speculation, a Unification may be possible between the 2. But with both being entities of their own (and both connected to Mother/Dark Elf) they came from the same body. So at least, a Parasitic unification is at least possible. With one being Merged with it's BioMetal and utilizing the power and ability of the other.

Zero - Omega - Model O.
_____\ Reploid Zero - Model Z.

Omega being body, and Zero being mind/consciousness... A permanent unification is impossible, since that is only reserved for how the Metal is forged. One BioMetal takes one Cyber-Elf. 2 cannot be contained within 1.

Ultimately it is speculation, but it is narrowing the line for possibilities.

________

And function of the BioMetal.

To speculate, we know that BioMetal must reach for a BioMatch. They must be together for the process to work. But I don't think that was the initial intention. Ceil was trying to recover any data about Zero. And stumbled across him, X, and the Big 4 within the Cyber Network. She wanted to attempt to revive them, with such means impossible. Until she discovered Model W. With it's throbbing consciousness. Analyzing that she came to discover a way to fuse a Cyber Elf into a physical object. That object must be some sort of BioMaterial forged within that era of technology. Since BioMechanical unions are common. Especially with Biological and mechanical components.

Since the BioMetal reacts to 2 aspects of it's host:

-1: Mental similarity between the Match and the BioMetal consciousness.
-2: Physical compatibility.

That is why the hosts are selective to only 1 person at a time.

It behaves with the flesh of the host, by decompressing itself and merging to the host's body. Whether it takes place internally, or solely externally is yet to be foreseen, but when this occurs, the 2 become on. The Biometal in a sense becomes the host's consciousness, and the host becomes the body of the BioMetal. It is more or less, a mutual exchange.