Eva:
If you mean that you want to open another .pln file while keeping the current one open, then when you use the "Open..." command, check the "Launch a new instance of ArchiCAD" in the lower left corner of the dialog box. If you mean something else, then post more information about your issue.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David