Here is some interesting info on the idea of a second OD on OD automatics.
Ford Auxiliary Overdrive unit for Automatic Transmissions | Gear Vendor Overdrive - Drivetrain
The three lesser features to this conversions, First is that Gear Venders is only a 22% overdrive, the Borg Warner is a 72% over drive.
Second it the $3000.00 price.
Third there has been some questions to it durability.
Rich