Can you place buttons above each other in a horizontal toolbar?

I'm trying to have one big button with two small buttons on its right side above each other. Is that possible?

No, not possible.

That explains why I can't get it working xD
Do you plan to add that possibility? Should I request that somewhere or something?

Maybe one day, but not in the near future, with the amount of other work in our queue. I think it would complicate things a lot, in terms of toolbar icon sizes as well as layout, both in terms of the code and for users.