New Post: Multi-valie metadata field

Not in the way you're asking. You can have a DataIdMultiSelector widget, that is a widget that allows you to select multiple items for a single field, but logically its stored as a comma (,) seperated list of id's in your datatype.

Read more about it here http://users.cms.orckestra.com/Superuser/Data/A-Guide-to-Creating-Datatypes/Widgets and search for DataIdMultiSelector.