Is it possible to tell Unity to copy two .dll files I have located outside of the Asset folder and place them where the executable file is located?
Do you mean during a build?
You can hook into the build pipeline and do it.
It is best to place any external libraries (DLLs) that your project uses into a “Plugins” folder within your project and they will be automatically included with the build.
If this is not possible for you (for one reason or another) look into copying, moving, and deleting files for c#. Here is a link to the MSDN that explains it: