How to populate a multi select field options based on options selected in another multi select field

How to populate a multi select field options based on options selected in another multi select field

For instance, 
If i have a multi-select field "MultiSelect1" with options "Option1" and "Option2", 
And if both options are selected, How do I populate another multi-select field "MultiSelect2" with options that are associated with those two options

ex : Option1 -> "alpha", "beta", "gamma"
      Option2 -> "apple", "banana"

MultiSelect2 should have Options ->  "alpha", "beta", "gamma", "apple", "banana"
if both options are selected in MultiSelect2