Rick,
Thanks, but I kind of already knew that. What we're seeing isn't "correct" behavior. Yes, it might (just maybe) be server side, but I kind of doubt it. It isn't that I'm asking how we can have two folks opening the same file, but I need to know how to avoid the problems we're having when somebody accidentally tries to do so. Read on - see if this makes the problem more clear:
The issue is that if the Mac has a file open and then the PC tries to open it, we do not get the correct message saying the file is already in use. Instead, the PC tells us it is a file created with the wrong version of Vectorworks. The PC, of course, will not open the file. What's really a problem though is that no subsequent files can then be opened on the PC until it is rebooted.
If we try the issue with two PCs (taking the Mac out of the picture), everything works as we expected it to.