MultiSelect OptionSet values based on selected OptionSet value

Hello, 

I have a requirement:

There is one OptionSet field(Branch) with one MultiSelect optionSet(Subbranch). When 1 value of Branch is selected, I need to display the related values in a Subbranch.

I found many links for Cascading 2 MultiSelect OptionSets (using Javascript & JSON) but in my scenario it is 1 OptionSet and 1 MultiSelect OptionSet. 

How can this be achieved? Any pointers would be of great help.

Thanks,

Manu.