Several editors can be used simultaneously, even on the same contig. In this latter case, it is useful to remember the distinction between the data and the view of the data.
The contig editor is a view of 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 this contig. You need not use undo in the same contig editor that the change was made - any view of this contig will do.
When quitting the contig editor, if changes have been made, you will be asked whether you wish to save the changes. Note that things may not work as immediately expected when using several editors on the same contig. 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" simply shuts down the editor without saving. If there is no other editor for this contig then your changes will be lost, otherwise the changes will be retained until the last editor for this 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, assume we have 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 to now be viewing the correct positions in the new contig, even though they're both now viewing the same contig.