Both products allow you to connect to SAP BW InfoProviders and other datasources, each one with its own functionalities and characteristics. This has led many users to use both tools together.
Now the new version of Analysis Office, the version 2.1, brings with it the EPM Add-in integrated with it.
Analysis Office and EPM Add-in are still separated in two different tabs (BPC DataManager functionalities are still delivered in a third separate tab), but all three tabs are more compatible than ever.
There is one important point to call attention to, this version of EPM Add-in (EPM plugin) which comes together with Analysis Office 2.1 does not support the EVDRE function.
Please review this great blog and note for further information:
The Official SAP Help guides are located here (See User Guides - Analysis Plug-in / EPM Plug-in)