This has been done many times for many, many years. Farmers and do it yourselfers used to build vehicles to use around their property using scrap parts, and for extra gear reduction, would mate two transmissions together. Many guys have done it for trail 4x4s (and even street trucks) for extra gearing. I wouldn't mount either transmission backwards though...for extra overdrive, put both transmissions in OD and you get double overdrive. For deep reduction, put both in low. With practice, you will learn to shift both transmissions together for close ratio shifting.
For a transmission that is built just for this, do a search for "brownie box". There are 2, 3, and 4 speed aux. transmissions that mount behind the main transmission. I bought one for my Ram to go behind the Spicer 5-speed. Its a 3 speed with overdrive (and the Spicer is OD), so I get double OD and some serious reduction. Yes, there are versions small enough to put in a 3/4 or 1 ton truck. My aux. would even fit in a 1/2 ton.
Jim