Multiselect text field choices to automatically match with another multiselect field from another table

I have a request table which through automation will created child record tables based on information in the request record... 

Ideally, I want to have multi-select fields with the options across the parent table and the child records.

When I go into the settings of the multiselect and choose "options from another field" the multiselect fields aren't there as a source field.

Any insight would be greatly appreciated.