Several editors can be used simultaneously, even on the same contig. In the latter case, it is useful to understand the difference between the data and the view of the data.
Each operating Contig Editor is a view of the data for a particular contig. With two editors viewing the same contig, making changes in either will effect the data that both are viewing, hence the change will be visible in both editors. Similarly, using Undo in either will undo the changes to both.
When quitting and saving changes, other editors for the same contig will act as if a "Save Contig" request has been made by using the "Commands" menu (ie changes are written to disk and the undo information will be reset). Answering "no" to the "Save changes" query, simply shuts down the editor without saving. If there is no other editor for this contig then the changes will be lost, otherwise the changes will be retained until the last editor for the contig is exited.
Interaction between Contig Editors and Join Editors is more complicated and generally isn't advised. However such interactions work consistently with the notion of views of contigs. For example, suppose there are two Contig Editors open on two separate contigs, and in addition to these a Join Editor displaying both contigs. Making the join in the Join Editor will update the two stand-alone Contig Editors so that they are each viewing the correct positions in the new contig, even though they're both now viewing the same contig.