(I split this from the original discussion)
The number of simultaneous users is a guide figure and is based on the server resources. If you have more than one Moodle on the same box then the resources available is split between them. Therefore, the number of simultaneous users is "shared" between them.
For example, if you think your machine can handle 50 simultaneous users and you have 2 moodle sites on the box then that's 25 each.
If you run other stuff on the box then that reduces the amount of memory available to Moodle. If you use a different web server it may mean there is more memory available. Etc. etc. etc.
One important note. I don't want to get bogged down in this. The number of simultaneous users is intended as a *guide* to the amount of hardware you need. It is not very scientific. It depends on how (well!) your server is configured, what the OS is, what the users are actually doing, what the phase of the moon is etc...
At the end of the day, you need to be measuring what is happening on your server...