Resolution:
TIBCO.WCF.DLL 2.0.0.0 has a dependency on TIBCO.EMS.dll 1.0.700.12. The dependency can be confirmed by running the following command in the Visual Studio command line tool.

ildasm /text C:\tibco\ems_wcf\2.0\bin\TIBCO.EMS.WCF.dll

Output:

====
.assembly extern TIBCO.EMS
{
  .publickeytoken = (5B 83 DB 8F F0 5C 64 BA )                         // [....\d.
  .ver 1:0:700:12
}
====

After EMS 8.x is installed, the WCF application may fail to start with the exception below.

Unhandled Exception: System.IO.FileLoadException: Could not load file or assembly 'TIBCO.EMS, Version=1.0.700.12, Culture=neutral, PublicKeyToken=5b83db8ff05c64
ba' or one of its dependencies. The located assembly's manifest definition doesnot match the assembly reference. (Exception from HRESULT: 0x80131040)

Or

Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly 'TIBCO.EMS, Version=1.0.700.12, Culture=neutral, PublicKeyToken=5b83db8ff05c64ba' or one of its dependencies. The system cannot find the file specified.

To resolved this issue, use either of the following.

1).  Import policy.1.0.TIBCO.EMS.dll under <ems_home>\bin into GAC.

gacutil /i <ems_home>\bin\policy.1.0.TIBCO.EMS.dll

When DLLs are loaded, this assembly redirects the reference from 1.0.700.12 to 1.0.820.7.

2).  In the app.config of your application, add the following section.

=====

<configuration>

  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="TIBCO.EMS"
          publicKeyToken="5b83db8ff05c64ba" />
        <!-- Assembly versions can be redirected in application,
          publisher policy, or machine configuration files. -->
        <bindingRedirect oldVersion="1.0.700.12" newVersion="1.0.820.7" />
      </dependentAssembly>
    </assemblyBinding>

  </runtime>

...

=====