It's an additional effect beyond just controlling the animal. Normally that would incur +1 magnitude even without any additional requisites. But if you want to look at requisites, you still have Intellego included for that. (The Familiars section of ArM5 has a spell specifically designed for this sensing, and it's not a trivial spell on its own.)
Hmmm... I just checked ArM5. You said "senses." Noting that InAn mental effects are 1 magnitude below InMe effects and seeing a base of InMe15 for a single sense with a familiar, I would put the base for a single sense at InAn10. How many senses do you want to share? It looks like the InAn base may be noticeably higher than the ReAn base.
If this is via spells, two thoughts:
Make two separate spells; it's probably easier.
Make R: Touch to lower the level and use Opening the Intangible Tunnel if you're decent at Vim (clearly fine at Rego).
If this is for a Greater Power, keep them together.
Reminds me of a 4th ed nonmagical Mercere I played loooong time ago...
In exchange for the loss of his left eye, he gained the power to see through his magical animal companions (a hawk) eyes.
The hawk was responsible for the loss...
No spells involved. We handled this through a couple of merits and flaws.
If only I could remember wich movie I stole the idea from
[strike](It's neither Ladyhawke nor Beastmaster...)[/strike]
There are InIm example on Hermetic projects about see all the surroundings. And, if the animal is the Magi Character, i used the guidlelines and examples from A&A with InMe (that goberns any intelligent being, including beasts with Intelligence).
I'd say that perceiving the world through an animal's senses (including perceiving the animal's drives, emotions, pain etc.) falls squarely within the "read an animal's surface thoughts" guideline, InAn10. Fully controlling the animal is ReAn15, so that's the base effect. At R:Arc, D:Conc, it's a level 45 Re(In)An effect: Base 15, +1 Conc, +4 Arc, +1 Intellego.
This is exactly what is done in the Familiars part of ArM5, though with InMe and so one magnitude higher. However, ArM5 states that determination is for a single sense. Based on the scaling of InMe and InAn, it would probably only be one magnitude higher for all the senses, though. That still wouldn't rise above the other base of 15 (in which case no problem), though it might depending on how an SG rules adding more senses.
Hmm, frankly, I think you may be extrapolating too much. It seems to me that perceiving through all senses is still level 10, by a strict reading of the guidelines. You are perceiving all surface thoughts. Otherwise Opening the Tome of the Animal's mind would only allow you to recover memories of one type: visual, olfactory, emotional etc. And it's not the case.
Note that the Familiars part says that they eyeball sharing one sense at level 15, by analogy with the guidelines.
It does not say that sharing multiple senses would be any higher. It could well be that sharing all senses is no harder than sharing one (in fact, you could argue that sharing only one is trickier, and requires a +1 modifier akin to T:Part).