Access to assemblies during development in Blend 4

I get an exception from my code when developing in Blend 4.

I narrowed down the problem to loading a specific library. Other libraries can be loaded perfectly, but this fails. So for this code:

var a = Assembly.Load("lib1");
var b = Assembly.Load("lib2");

An exception will be thrown in line two: the file or assembly 'lib2' or one of its dependencies could not be loaded. The system cannot find the specified file.

If the same code runs outside of Blend, it does not throw. It seems that both assemblies have the same path in the project, and both are marked Copy Local.

Any suggestions to fix this problem?