In IIS6, this was a pain area. On a 64-bit Windows 2003 Server, you cannot run worker processes in both 32-bit mode and as well as 64 bit mode. Either only one of them was possible. This was possible by the below Metabase Key which would be applied to the W3SVC/AppPools node.

W3SVC/AppPools/enable32BitAppOnWin64 : true | false

Read more how to do this in IIS 6.0 here. But, in IIS7, you can run 32-bit and 64-bit worker processes simultaneously. Let’s see how to do it.

You have the same enable32BitAppOnWin64 property for the applicationPools in the applicationHost.config. Now, you can set this for individual application pools. Below is my sample configuration:

Full Article

IIS, ASP.NET, Microsoft, Web Server, How To