I ask because of the existence of polymaths, people who are very good in several fields of endeavour.
Let us limit this discussion to saying a character can have either 1 MMF, or up to 3 non-overlapping mMF. Or at least minimal overlap.
So that a Tremere or Mythic Blood might have another interest apart from the one their ancestors loaded them with
Your character can have two Affinity with Arts, and two Puissant Arts, more than one Special Circumstances. So why not a couple of minor focii?
I'd separate the answer:
from a game point of view? the multiplie +lowest art combinations already exist for characters with chthonic magic, diedne magic and focus so having 2 or more appliable focus do not bring anything new.
From a story perspective, I like the idea of "one" focus, to entertain the idea that you are... well focused.
I'd rather have a character with a double focus on the same area (if need be) rather than have 2 focuses on different areas. The "double focus" keeps the theme.
However, if twilight happen, I already saw focuses go from minor to major. Eg: creating visual species (minor) becoming visual illusions (major).
Why? Because it's RAW. But I suppose that's not the answer you're looking for. From a different perspective, one could argue magical focus tends to outweigh the benefits of other virtues such that a character who has several might be overpowered. But tbh, I'd be open to show leniency to a Tremere who wants to pick Mythic Blood, and I've had tables where the restriction only applies to character creation, and not initiation or Twilight. On the other hand, Potent Magic exists explicitly to comprnsate the limit on Focis.
Why would you now allow two MMF if you allow three mMF? MMF is barely better than mMF, but you pay 3x as much for it. Why? Because you build your magic to use your Focus as well as possible, which means even a Minor one that is chosen well can cover a huge portion of what you do.
Now, that's not just a point about that limit, but about the whole thing in general. Keep in mind that with one mMF, almost everything a character wants to do can be covered well. Throw in a second and there isn't much left. I'd probably never take a third, as almost everything I would ever do would be covered by one of the first two. So expect that people will almost always have a mMF that applies to whatever they're doing. If you're fine with that, go ahead and house-rule it.
Fundamentally because they aren't really a focus if you have more than one.
Mechanically because it is too easy for them to overlap, even if that is not the intent.
You can create areas of expertise without magical foci, and this can include multiple areas. A certain degree of overlap even makes sense from this perspective- craft magic and necromancy would both benefit from a good rego score as example, and you could probably select a few other overlaps depending on what kind of craft magic you might be interested in...
I must confess it does seem that people are leaning on the linguistic crutch of "focus" excessively, while ignoring synonyms such as "Affinity", "Special Circumstances", and "Puissant".
I am not entirely blind to the point.
Still, the idea of, say a Tremere with a House mMF in Certamen, a Mythic Blood mMF in Healing, and a Personal mMF in, for instance, Ice does have appeal.
Perhaps that is another way to look at it?
Either have a single Major Magical Focus that precludes any other focus,
or choose up to 3 minor Magical Focii each from a different category:- potentially one from your character's Personal idiosyncratic magic, potentially one from your enforced House lineage, and potentially one from ancestral Blood connections (eg Mythic Blood).
Vaguely similar to Virtues & Flaws come in categories like General, Story, Personality, etc
If 2-3 minor can be thematically linked in a major, i'd allow it. There are already example of those in the Mysteris about the zodiacal signs and planets (major foci).
But in anyway, "certamen" as a focus is purely artificial and I wouldn't include it in any such major-gathered focus.