I've heard of one spell that worked for two people, one I know personally. Whether they really work is up for debate. Haven't tried it (already have my familiar) so your guess is as good as mine.
The idea that a pet is the same as a familiar came from the centuries when poor women who had only a pet for company were often accused of being Witches.. It has continued in modern times because of the crop of poorly researched books on the market these days. But the fact of the matter is that our beloved pets and a Witch's familiar are very, very different things.
A Witch's familiar is an animal spirit or a magical construct created by the Witch or the Magician as an adjunct to their magical workings, as a repository for power, and if needed as a defense against magical attack from another magic user. A Witch or a magician can draw upon the energies of the familiar when they need an extra boost of energy in a magical working. Conceivably if the spell is important enough, the Witch could completely drain the familiar of energy causing it to "die" or cease to exist. And in the case of a magical attack, the attack could be diverted to the familiar so that it would be the familiar which is destroyed rather than the magic user. None of these are things that I think any of us would want to do with our pets.
There is an excellent article on familars here: http://en.wikipedia.org/wiki/Familiar_spirit
Animals are very sensitive to the energies around them and they may be drawn to us when we are working magic because they can sense the energy. They also want to be around their people, which again will make them want to be where we are..and that may be where we are doing magic. Neither of these things makes the animal a familiar or inherently magical in and of itself.
There are three types of familiars.
Astral, physical, and artificial.
The astral represents more of the "totem animal" concept. This familiar will greet you through meditation and workings. This familiar also can reveal itself in a certain animal form through such states of mind. This familiar carries out tasks that give it through other planes.(ritual workings, and the such.)
The artificial familiar is one that you create yourself, to do the similar tasks.
The physical could be things as a pet, but you cant send it through different planes to carry out workings.
What most people don't understand is that when working with the familiar, its a "give and take" relationship. Its not a "master and servant" relationship. The astral needs you to do things as well, and they could even become a bit rebellious. The artificial needs energy, and to be charged in order for it to carry out tasks. And as for the physical, you still have to feed the poor animal, give it affection as it does you.
