Currently, there is no functionality available which allows user to fill one multi select field based upon the another multi select field dynamically.
Solution:
We have a tool which does the same. Suppose there are two multi select fields named Category (i.e. pers_category) and Sub Category (i.e. pers_subcategory) under the Person Entity.
The Selection options for Category field are A, B, C, and D and the selection options for Sub Category field are A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, and D3.
So, now If user select A from the Category field then the A1, A2, and A3 will get filled in Sub Category field. Since this is a multi select field, user can select more then one value in the category field.
If user select A and C from the Category field, the corresponding options (i.e. A1, A2, A3, C1, C2, C3) will get filled in the Sub Category field.