I don't see what the problem would be with wearing a cummerbund under a double-breasted jacket. That's how I wear mine. Are you concerned it will make the jacket too tight? In my experience, the cummerbund doesn't add any notable thickness. In fact, it makes everything underneath the jacket more streamlined.hectorm wrote:I would advise against a cummerbund with a DB DJ.
A trick that works for me when I'm wearing DB jackets with no vents at dinner is to undo the outside button and keep the interior button fastened. You'll see that It opens up nicely and you're not showing the meeting of trousers and shirt to anybody.
I'm not sure what unfastening only the outside button accomplishes. It seems you'll just be left with the front free to swing about, while you haven't made the jacket any more comfortable to sit in.
For reference, here's my double-breasted diner suit worn with a cummerbund underneath: