Also, I'll like to point out, as much as 'Calyrex' is "an edge case" here, it really isn't.
(this isn't a response to you Poke J btw, I'm just quoting you to explain something in general)
Think of it this way, let's ignore the text, and the name, that shows up on screen for a second.
Shadow Rider Calyrex (+Spectrier) and Ice Rider Calyrex (+Glastrier) only has one ability respectively.
However, in the game files, that ability is meant to have multiple effects, which isn't a super rare thing if you think about other abilities.
People just get confused cause (a) it appeared to be 2 separate abilities before fusion, and (b) the ability has the same name between both Calyrex's. So it gives the impression it is doing some variable ability stuff.
But that isn't the case.
The effects are the ability are set in stone. Heck, they even have different IDs. Stuff you normally can't see if you don't look at game data. It has different IDs.
This ability was developed with the 'abilities that appears to have fused' gimmick in mind; it was made to look like it can use both abilities. So to answer back the original question:
1. No, you cannot have 2 abilities active at one time, and to answer the bonus question (that may appear) in relation to Calyrex:
2. No, you cannot manipulate 'As One' to have different abilities bundled in it. It's hard-coded to appear as though it's Unnerve + one of the mount's ability. (which one depends on ID)
If you want to change the effect, you gotta ROM edit, not SAV edit.
Tho at that point, might as well just edit Rock Head directly?