Customer is converting an application from Net Express 5.1 to Visual COBOL 2.1 but they still need to maintain their existing Net Express application.  Both Net Express and Visual COBOL products are installed on the same development computer.

Each version of the application must maintain its own set of sources and copybook files in separate folders.

How can the environments be setup so that the COBCPY environment variable points to the correct copybook folder for the product being used?

You can set COBCPY and other environment variables in the computer's environment to point to the default Net Express folders.
You can then create a .bat file that is executed to start Visual COBOL that will set the desired environment variables prior to starting the Visual Studio IDE.

Setting COBCPY in a .bat file prior to executing devenv.exe will override the value set in the control panel as long as you don't include the %COBCPY% as part of the value.

This will work:

SET COBCPY=c:\\vcobol\\cpy
devenv.exe

This will not:

SET COBCPY=%COBCPY%;c:\\vcobol\\cpy
devenv.exe