Wow nice bandwidth you are pushing through Paul! You mean two Raspi 2's sharing an Internet connection, each relaying 27 Gbytes per day at 5.4 Mbit/s on the average?? Total 10.8 Mbit/s?? Or 2.7 Mbit/s each?
It is just 1 single Rasp2 - running 2 tor instances on 1 IP, details here https://gitweb.torproject.org/debian/tor.git/tree/debian/tor-instance-create...
Any specific reason you have for running 2 instances of Tor on the same Raspi instead of one?