And the perception like the only Charasteritic is not true, i've found spells with Inteligence + Finesse stress rol (Magi of Hermes two spells), one related to memory and others related to imagination and experience (and on one spell Dexterity - pages 99-10 the Petalichus' spell Unmake this Cloth). True, Perception is needed to Aim, but my doubts are not concerned to the aimed spells, or fast castign taht uses Quickness; if not Rego and Creo craft magic and really good Illusions.
That'd be how to teach the ability, not how to make a good strike now 
I might allow a a spell to average the abilities of teacher and student. This would represent the teacher showing student how to do things but student inclinations would affect things. This could be a teaching tool allowing for a slight bonus to teaching or training totals.
Yeah, fine, but my spell try have points on common with Wizard's comunion and the effect of boost the pentration like Hughes of Flambeau from MoH on his Talisman. One helping spell.
Yes.... to teach you how to do the moves.
But to actually PLAY tennis, where you have to run all around? This won't be efficient at all.
Even if that was possible, the student + master would never be more efficient than the master alone... Which is exactly what you get when you add finesse.
This is why I said I would allow a spell that averages the finesse of teacher and student. I would give a +1 or +2 to study totals for skill the spell is designed for when doing training.
In addition to study bonus, I would say it means you can apprentice's sigil for casting the spell while using Master's finesse. Good way to keep blame for crimes off your shoulder and on apprentice or fellow mage. Esp if you cast it such that person is not aware of their actions (ReMe control).
Hum...
The sigil... Well, the original spell should display its caster sigil, while the new one should display the other magus's sigil.
So... Well, both sigils should be displayed at once, IMO.
But then, the second character can use Vim to hide his sigil 
I insist, this is not one teacher spell, the Apprentice on the spell name is just like the Infernal on some spell...
This is one way to help to others magi... just that. To learn, Teach and normal Practice rules are enough. This is simply Metamagic, inside of the normal rules, just like the Muto Vim spell from Hermetic Projects emulates the Minor Virtue of Tethering Magic or the Mastery Ability. The Precise Casting Mastery Ability is the metamagic thing that i amulate with this spell.
The main difference between the spell you've outlined and Wizard's Communion is that WC is cast by both wizards, while yours would only be cast by the one with the higher Finesse. Adding two Finesse skills together would be much less common an idea than joining powers to cast a big spell -- WC comes from ancient Mercurians, and does not fit well into Hermetic theory. Adding two Finesse scores is also more powerful than adding Techniques and Forms, because Ease Factors are such a small range.
For a single casting of a spell, to go from an average roll of Int 2 + Finesse 3 + die (avg 5.5) = 10, to Int 2 + Finesse 3 + Finesse 3 + die (avg 5.5) = 13 is a big boost.
I haven't read MoH yet, but speaking of Talisman Attunements....most attunements give a +2 bonus to Casting Total, and this is considered powerful magic.
Again, I think that, as two different spells, both your suggestions would work by the rules. I just don't know if I would let the "combine Finesse scores" spell into my game. If you were to make it more like Wizard's Communion, and both magi must cast the same spell at equal level, to join efforts, then I would be less anxious about putting it into my game.
The attunement of MoH is not to the casting, it's this:
Magi of Hermes, page 51
So... +15 to Penetration with a 20 MuVi effect...
I am geting +3 to Precission. + 3 To precission can be granted by: +30 PXs to Finesse, +15 PXs to Finesse and one Speciality on your favorite use, +5 on Finesse and Speciality and one Mastered Spell with the speciality Precisse casting. And precisse casting is better, because reduze the botch dies, my spell not. I call the spell like "risky business", becaus you can have "high" bonus but it still bieng just like ever of risky.
Ah yes, the infamous penetration boosting effect.
If I could remove one effect from the whole book, that'd probably be it.
That wouldn't change all my point.
Without that effect, Muto Vim can make one Diameter greater to Duration increaze Moon (+10 levels and one period on the Lab). So my change is little on that.
From another thread, it has become clear to me that a large part of my problem with this is the general utility of the Finesse skill.
I feel that the skill system in ArM is very unbalanced to begin with, with skills like Finesse being extremely useful, while other skills (languages, prof:scribe, Cult Lore, etc) rarely useful.
Well, i think that all Abilities are usefull, but the utility factor depends totally about the situation. See, Computers (on reality and others games), very usefull to search information and to work, but not on the Amazon Forest without one single tech piece.
Finesse is one great abilitie, by that it is an Arcane Ability, where others are mundane; with the cost of special background or game experience you got one great power.
Coming back to my spells... One possible need to my spells is one Rego requisite...