I happen to stumble across this, guidlines from the DOT regarding transport of service animals in accordance of the ACAA:
http://airconsumer.dot.gov/rules/20030509.pdf
What About the Passenger Who Has Two or
More Service Animals?
• A single passenger legitimately may have
two or more service animals. In these
circumstances, you should make every
reasonable effort to accommodate them in the
cabin in accordance with part 382 and
company policies on seating. This might
include permitting the passenger to purchase
a second seat so that the animals can be
accommodated in accordance with FAA
safety regulations. You may offer the
passenger a seat on a later flight if the
passenger and animals cannot be
accommodated together at a single passenger
seat. Airlines may not charge passengers for
accommodations that are required by part
382, including transporting service animals
in the cargo compartment. If carriage in the
cargo compartment is unavoidable, notify the
destination station to return the service
animal(s) to the passenger at the gate as soon
as possible, or to assist the passenger as
necessary to retrieve them in the appropriate
location.
What if the Service Animal Is Too Large to
Fit Under the Seat in Front of the Customer?
• If the service animal does not fit in the
assigned location, you should relocate the
passenger and the service animal to some
other place in the cabin in the same class of
service where the animal will fit under the
seat in front of the passenger and not create
an obstruction, such as the bulkhead. If no
single seat in the cabin will accommodate the
animal and passenger without causing an
obstruction, you may offer the option of
purchasing a second seat, traveling on a later
flight or having the service animal travel in
the cargo hold. As indicated above, airlines
may not charge passengers with disabilities
for services required by part 382, including
transporting their oversized service animals
in the cargo compartment.