32-bit versus 64-bit ODP.NET to Oracle connection
Did you control PATH Environment Variable to which comes first, 32 or 64. Which ever client is installed last,  change PATH accordingly. I have encountered similar issue. Windows uses PATH environment variable to decide which dlls to load. I have two clients installed in my machine and used PATH variable to force which one to use.

C:\oracle\product\11.2.0&gt;dir
 Directory of C:\oracle\product\11.2.0

20.09.2011  10:07    &lt;DIR&gt;          .
20.09.2011  10:07    &lt;DIR&gt;          ..
20.09.2011  10:08    &lt;DIR&gt;          client32Bit
20.09.2011  10:04    &lt;DIR&gt;          client64Bit

C:\oracle\product\11.2.0&gt;echo %PATH%

XXXX;C:\oracle\product\11.2.0\client64Bit\BIN;