Hi, thanks for posting!
This can't yet be controlled at assembly level, but it most certainly can be controlled at fixture and test level through the use of attributes such as
ExclusivelyUsesAttribute. The documentation has some detailed information on
ways that you can control this.
A future expected feature is to allow the concurrency attributes to also be expressed at assembly level, where they would apply to all tests within the assembly.
Cheers,
Remco