You are right. As MashZone's license model is based on named users it is forbidden to log in using the same credentials twice.
Even further--currently there is no multiple window support. So if it works in your office you are lucky.
Please note: Once you changed the number of columns, their name or data type of a data feed's output it will affect the data assignment. E.g. a feed's column Region is used to render a column chart's x-Axis. Once you changed its data type, i.e. String to Number, viewing the chart triggers a runtime error. A yellow triangle will indicate this.
Just as I thought. Thank you for the quick reply.
I always pay attention to the changes made to the feed. Usually I extend the feed with another column so it doesn't make much trouble to the data assignments.
Hope multiple window support will be implemented in next versions. Single window is rather boring and inconvenient. One can't even view two different mashups simultaneously, e.g. for compare.