If both barrels were already cocked on the Franchi they would just fall open just like almost any other o/u. What do you fear about an easier opening o/u, I would call it a plus.
In most cases a o/u with both 20 and 28 barrels will be made on the 20ga. frame so when you put the 28ga barrels on you are basically still shooting a 20ga with expensive shells.
In o/u you can never go wrong with manufactures that start with a "B" Browning/Berretta
I wouldn't overlook a lightly used name brand(higher end o/u)
If you aren't a high volume shooter the Mossberg might work but I wouldn't count on being able to get parts 10yrs down the line, then again you might get luckky.
