Could we look at adding a toggle/setting option on multi-select field types where if "on" a new form submission does not overwrite the existing field value stored, but updates the existing value to include the latest submission.
Liz Gibbs has requested this: Want to achieve this scenario : 2 forms. Form 1 adds X to a multi select field, Form 2 adds Y to a multi select field. Want X,Y in the field not just the last value entered.