select top barrel,open gun,pull and hold trigger.Shut gun and you have released top hammer.Now pull trigger to trip bottom barrel.If you do not have o/u barrels it's simple to remove the hammer spring.
I think I get what your asking. Don't be dropping the hammer on the top (plugged) barrel. I'm sure you could machine out face to allow a brass headed plunger into the barrel. However, I like to remove the trigger group to inspect the springs after shooting. That's the time to de-cock the hammers. If I'm lazy I just take the barrels off and depress the top lever pin. I then use a single snap cap and push it against the breach face on the lower barrel and fire. Flick the safety on then back off. This allows me to fire the top barrel after I slide the snap cap up to that position on the face.
The top barrel n a unsingle should be blocked of as a safty issue. I man at a shoot dropped a shell into the bottom barrel of a top single and blew the barrel up . Iy may have been a custom barrel made from o/u barrels but when open it looked like a reg o/u very dangerous I would plug it even with a snap cap glued in place. An easy mistake to make.