RE: Guide to DB2 HADR Setup with Sametime 9 Ben Williams 3.Sep.14 03:45 AM Lotus Notes Administration 9.0All Platforms
ACR should be sufficient. I haven't configured HADR for v9 but the options available are probably the same and the theory of ACR should not have changed much between 7x and 8.5.5x.
You will also need to have a WebSphere proxy in front of both meeting servers, either stand alone or on the same node as the application server. I'm sure you have this already but in case you do not then you should create them otherwise fail over will not work and you miss the good stuff such as caching, forcing SSL etc.