Test-driving does not seem to allow the registration of more than one user 

Applies to vers 2.1.2, tested in standalone 

Test driving online registration will block after the first successful registration. It tells you that the user is already registered (which is true as you just have done it) but seems not to give any opportunity to select an other user.

To avoid this problem (will be fixed in 2.2), on the first registration do the following:
On the registration page change the Email address indicated (it is your own Email)  to any other valid Email. Now do your testdriving. As long as you never use your own email, you can change the Email on the registration page every time you do a new testdriving.  Clearly, you cannot register with an Email that has already registered for an event.

What to do, if you are stuck (on the registration page you receive an error and you cannot change the e-mail address):
Delete this participant (Go to search for participants, must select test participants specially otherwise they will not show up. Details of participant will show delete button) 

Happy test-driving