Set ""Use Alternate Label Text"" dynamically for a Text field

There is not a way to dynamically update an alternate label by form rules or by any other native means.  I suggest that you have a lookup field or formula field  and then contrive some words to tell the user what to enter into the two fields, or perhaps you can arrange the form so that these calculated label fields are close enough to the formula fields that have their desired names so the user understands how to use them.