AC10 explicitly supports running more than one instance of AC at a time. In fact, in certain situations you have to. Like pen sets, this is a case where a mandatory technical change has a lot of secondary benefits.
• With no PlotMaker, there's no more dreaded BGArchicad. Most drawing links in a project will be internal, but you can also have external links to views in other projects. These drawings are updated in a second, identical copy of AC.
• In order to merge layouts from a 9 layout book into a 10 project, the book must be open in a second instance of AC10.
Those are the requirements, now the pleasant side-effect: When you issue the New or Open commands, the dialog box has a checkbox for 'Launch a new copy of Archicad'. Who can't think of tons of reasons to have multiple files open:
• Work in one project while another one, under construction, is open in the background, so you're ready when they call with questions from the field.
• Copy and Paste between projects.
• Work on a hotlinked module without closing the host project.