RE: Map a global multiselect option set
Hi Emma,
Yes, it is possible.
First of all, when you say "Map", do you mean mapping in the relationship, or do you mean that whenever a second entity is created or updated, the value of its global multiselect option set field is always the value of the corresponding field of the first entity?
The difference between them is that the mapping in the relationship will only take effect when the second entity record is created from the sub-grid of the first entity.

If you mean the mapping in the relationship, then yes, it is possible.