Hi,
As per SRND
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/8x/uc8x/callpros.html#wp1155823
"On the other hand, Business Edition 6000 does support node clustering with one additional server and therefore can provide redundant call processing and registration services. Business Edition 6000 can be deployed with a second UCS C200 Rack-Mount Server (or MCS server) to provide high availability for call processing as well as other applications and services."
HTH
Manish