This is a great new feature in GP 2013. If you new to Dynamics GP – you can be logged into the Practise company (Fabrikam Ltd.) – and that does not count towards your concurrent licenses.
In the example below I have two users in the system (one in a live company, and the other in Fabrikam) – as you can see it is using up only one GP License.
For this to work – you need to setup the user as a “Lesson User” in the User Setup.
Finally if you are logged into multiple companies in GP at the same time – you would find it useful to be differentiate the companies by colour. You can do that using the SDT in a couple of clicks. Read more here - Differentiating Companies in Microsoft Dynamics GP