If you ever need to use two different DLL versions because a third party SDK requires a later version of DLL which is required to run your application or Umbraco
Firstly download the DLLs you require and place them in a directory structure in the BIN folder as follows.
Next remove any reference to the log4net.dll in any of the projects in the solution of the application.
Edit the Web.config to point to each assembly and version.
<runtime> <assemblyIdentity name="log4net" publicKeyToken="669e0ddf0bb1aa2a" /> <codeBase version="126.96.36.199" href="log4net/BazaarVoice/log4net.dll" /> </dependentAssembly> <dependentAssembly> <assemblyIdentity name="log4net" publicKeyToken="null" /> <codeBase version="188.8.131.52" href="log4net/Umbraco/log4net.dll" /> </dependentAssembly> </assemblyBinding> </runtime>