Side-by-side execution is the ability to run multiple 
versions of an application or component on the same 
computer.

You can have multiple versions of the common language 
runtime, and multiple versions of applications and 
components that use a version of the runtime, on the same 
computer at the same time.

Since versioning is only applied to shared assemblies, and 
not to private assemblies, two application one using 
private assembly and one using shared assembly cannot be 
stated as side-by-side executables.