I am wondering this as well, since the Managed Folders UI clearly has checkboxes, and a multiple item action menu. But when you select another checkbox, the existing one gets automatically unchecked, so only one can be checked at a time. This seems like a bug due to the rest of the features on this page.
Thanks for bringing this up. So that our team can investigate this bug with trying to select multiple shared folders, could you submit a bug report to our team following the steps on this page? https://support.logmeininc.com/lastpass/help/how-do-i-report-a-bug-to-lastpass