I don't know if this issue has been brought before, but according to ArM 5th a magus can only have a single magical focus, either major or minor (if he has any).
Now HoH:MC (I don't have TMRE yet) there are rules regarding Initiations which confer a Magical Focus (the only ones which comes to my mind now are those confraternities of House Verditius which initiate in Minor Magical Focus with Swords and with Ignem.
My question is, can a magus who has already a magical focus in some other area follow that initiation? Possible thinks that could happen:
Exception to the rule: that magus develops a second magical focus (this option would surely unbalance the game)
The magus changes his magical focus to the new one (again, perhaps a bit unbalancing but not so much as option 1)
The magus cannot be initiated, and can only join the cult as an amicus or collaborator or not at all.
Or perhaps I am missunderstanding the rules and this as already covered elsewhere (and I should get a copy of TMRE)?
As far as I understand it, there are two possible options that the Mystagogue and the initiate may try:
Changing the previous focus into the new one: this may be done by adopting the loss of the previous focus as an ordeal; the TMRE rules explicitly allow this option.
The character keeps his old focus, and gains the new magical specialty as a Potent Magic Virtue. You can have as many of these as you want. It seems that Potent Magic Virtues have been specifically invented by mystery cults as a way of bypassing the "only one ever" limitation of the Focus Virtues (even if Potent Virtues have spread beyond the contriol of the mystery cults, and now they may be possessed by any magus).
Much more detailed rules about Potent Magic may be found in TMRE, but while you wait for your copy to arrive (you have ordered it, right ? anyone that is seriously interested in mysteries should absolutely have both HoH:MC and TMRE), you can do with the abridged potent rules in HoH:MC.
Since this is a not too infrequent occurrence among initiates, it is rather likely that the cult, if it has been around any significant amount of time, has variant scripts to do either or both of 1) and 2). Otherwise, the Mystagogue would have to experiment, since you cannot change the Virtue (Magical Focus to Potent Magic) by varying the script. Unless, ... one possibly makes an exception, since Focus and Potent are quite similiar in nature ? Caribet ? CJ ?
Well, it all depends what TM:RE says on this issue, which I know Mark and I discussed at one stage. I'm pretty sure that this isn't a special Criamon power or anything, in which case, you can do the above, yes.
Now, there's a chance that Mark and CJ changed this in their book, in which case then the Criamon have a weird power. How this works is unclear, and the Criamon would debate endlessly if this meant a concious force was selecting their virtues, and who that concious force might be, or if they were selecting them themselves, or if this was an impersonal working of Necessity or something.
if you have a MMF, and a script which currently grants a MMF, then you have 2 possible choices in TMRE
Vary (use the rules in TMRE to Vary a script) and add a new Ordeal to "Lose the existing MMF" - then you get a bonus for the Initiation and you become free to take the new MMF.
find a different script which offers Initiation in Potent Magic (in the desired area) instead - most cults have such alternatives if they've been around long enough.
You can't do (2) with the Variation rules 'cos they specifically exclude "change the Virtue Initiated". On the other hand, Potent Magic was so closely designed as the alternative to MMF, I'd allow it as a Variation, much like the pseudo-Variation in Initiating PM or MMF in an Astrological House or Sign. (You can use either the Mystagogue's House/Sign or the Initiate's, and still count it as "Virtue known by the Mystagogue")
I have recently received TMRE, but not yet found the time to read it.
But the balance problem with more than one Magical Focus is the stacking issue. And that can easily be remedied by combining the new and the old Magical Focus into a single new one - which clearly will not stack onto itself.
So if Flammiferus had a Major Magical Focus (Ignem Damage) before and was initiated into the top notch mystery of the New and Reformed Wandering Asclepiads, a Minor Magical Focus (Healing People), he might - in an appropriately house-ruled campaign - find that he afterwards has a Major Magical Focus (Ignem Damage and Healing People).