Express Multi-select Attributes

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Hi,

I am keen to get stuck in to using the Express element of Concrete5, however I am having trouble finding out if it will be useful to me.

I am looking to add new user attributes by using express entities, however I want them to be multi-select and this doesn't seem possible so far unless I'm mistaken. I've got as far as creating 2 Express data objects with an association, then creating an attribute, only to find that I can only select one express item at a time.

Can anyone help shed some light or tell me how to do this?