If you have snap caps, put them in the gun with the barrels and forend in place. Pull the trigger to fire the first barrel, hit the recoil pad on a hard surface to reset the second barrel, pull the trigger again. Both springs are now extended.
Remove the forend and then open the gun, making sure that the barrels do not rotate too far as the forend no longer stops the barrels - or resets the hammer springs. Dump the two snap caps on a table, close the gun and put the forend on the gun. The gun is now ready for storage. When you open the gun the next time, the hammers will reset.
I do not drop a hammer on an empty gun as it will eventually damage the firing pin assembly. As Tom M notes above, you can also use a piece of hardwood or a copper penny also works if you hold it tight to the breech and trip the hammers with a reset blow in between. However, since I store my guns in a safe assembled, I prefer the snap cap approach.
Z
