Situation:

I have a fieldA of type List and having multiple-values option checked (this is being pre-populated by list from External table). I have another fieldB of type list that is a child of fieldA. Therefore the list values in fieldB will varry depending on what data I choose in fieldA. Now, when I select more than one data from fieldA (since it's a multiple value field), I would like the fieldB to display a combination of child values pertaining to the multiple data selected in fieldA. Please advise if this is possible and how.