Known as polycaudal (poly = many, caudal = tailed) cats, they should
really be called "bi-caudal" (bi = two) as they only have two tails.
Since cat lovers were already familiar with polydactyl (many toed) cats,
the name polycaudal stuck.  The trend to create "novelty breeds" based on
radical physical changes has led to concern among breeders and
veterinarians. This page summarises the information currently available.

Bicaudal mutations have historically occurred in Japan where two-tailed
animals have entered into folklore.  Bicaudality is therefore not a new
mutation.

Apart from arousing suspicion among superstitious individuals, the two-
tailed trait has not been shown to be disadvantageous. Most cats examined
had full control over both tails and could work them in unison, but not
independently i.e. both tails were raised, lowered or waved at the same
time, but the cat could not raise one while waving the other. Most
examined cats showed "tail dominance" where there was greater motor
control over either the left or right tail. Consequently, the dominant
tail tended to be larger with better muscular development although the
secondary tail was not actually atrophied ...

More at http://www.messybeast.com/polycaudal/polycaudal.htm