The B-73 Mack I learned to drive big rigs on only needed the clutch for starting and stopping. It had a 5X3 setup, with both transmissions in a common case. Mack called that a Tri-Plex. Of course, that one had a Cummins 262, but the RPM range was very similar to a stovebolt engine, and doing the RPM matching to slip the gears in and out of mesh isn't all that difficult- - -just don't rush the shift!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!