You use the same method that you would normaly use when accessing a VI within the same labview.
Also, when communicating to another executable then you need to specify "localhost" as your servername. VI Server uses the TCP/IP protocol. TCP/IP communications between two executables require that each end of the TCP/IP link has a different TCP/IP port number. When the two VIs are running under LabVIEW, they are within LabVIEW (in other words, not communicating between executables or separate programs).
You can change the port number in the ini setting for that executable. The .ini file should have the same name and be in the same directory as the executable.
A typical ini file after the port has been changed on the executable looks like this:
[MyExecutable]
server.tcp.enabled=True
server.tcp.port=3364
server.tcp.access="+localhost"
Also, one more thing. If you want to test your code before you create an executable, you can launch multiple instances of LabVIEW. I haven't tried this but it's shown in this post in the FAQ:
Can I run multiple instances of LabVIEW?