I have many versions of a library and want to compare the different versions of e.g. the main VI. If I open one version and try to open another version the former version disappears...
Right now, you can only have a single VI of the same name open at one time. You'll have to open one, rename it, and then open a second. Then, if you have the professional version of LabVIEW, you can use the Compare VIs tool to get the differences.
