Whatever initiates the starting process (PID) is going to determine the runtime used. So the first COBOL element to be used will load that (whatever it was compiled with) runtime into memory.
So if I have an executable for example is compiled for NX 5.1 any programs (.INT\GNT\DLL) loaded within that process MUST be compiled under NX 5.1, you can't mix them.
The only way,and I stand to be corrected, would be to spawn a new process that could use VS COBOL compiled files and perhaps pass it a file name etc in order to pass data across from something compiled in NX.
Both NX 5.1 and VS COBOL runtimes can exist on the same machine without any problem.
Neil