Multiple NUnit test assemblies, each requiring different config. How to get NUnit to run them all at once

I have 13 separate but related architecture assemblies, and 13 separate NUnit test assemblies, each one containing all the test fixtures for its matching architecture assembly. I am using NUnit 2.5.2 (latest version currently).

I can run each test assembly separately in the NUnit GUI and all the tests pass. However, when I come to combine them into a single NUnit project file, NUnit insists on applying a single config file to the whole test run. This won't work because each test assembly requires different config. I can't merge them into one "uber-config" file because some of the sections are mutually exclusive. I have tried running each assembly in the project in separate AppDomains, and also separate processes, but in both cases it fails to use the DLL-specific config file, so all the tests crash and burn.

I have done a Google search but so far I have not found any indication that NUnit supports this scenario. Am I right, or have I missed something?

I have tried my hardest to re-architecture the tests so that they could share the same config file, but I've had to admit defeat on that front.