Right now a Multi-select Picklist could only be on the dependent field. There is a custom way to use it as controlling field?
Right now my workaround is to use a validation rule to avoid the update/insert of wrong pair of values.
Is it "possible"? Yes, but the amount of effort to create and maintain this feature would far outstrip any possible usefulness it might have. You'd have to build an entirely custom Visualforce page or Lightning component, plus presumably one additional component of either type to render and select values, plus a page or component to help configure the metadata without importing raw data, and so on. I could easily see this project taking tens of hours of development work to get it just right. Of course, you'll have to choose if the tradeoff is worth it, but I don't think I could justify that much time spent for such a small feature.