In a future version, I am considering adding the following features:

• Stack re-ordering via drag-and-drop.
  Allows the order of stacks to be changed when using the stack_multi_select form field.
• Prevent stack duplication
  Prevent a user from selecting a single stack multiple times when using the stack_multi_select form field. 
• Add stack maximum variable
  Add a variable that allows a developer to specify the maximum number of stacks that can be added when using the stack_multi_select form field

If any of these features or something I missed might help you, please get in touch with me and I'll see what I can do.