Ever drive a Mack tri-plex? A double clutched 3rd low to 2nd hi isnt too hard even for an ameture like me.... unless you need to have hands on the steering wheel Ive made a few different low/hi shifts while moving, Im sure some one will scream at me for that but it works .

 

Yeah...I've driven the old trucks with the auxiliary box...you could hang your arm through the wheel and grab one of the shifters...but I'm not certain this synchronizes that way...could be wrong though...

 

Back in '68 I got to sit onna bucket inna triplex shifted Mack...shotgun seat was gone. The poor driver had to shift through 60 miles of backroad taking a steam traction engine to a Steam Show. The lowboy didn't hava winch so the engine was loaded under steam pressure, then after we got going, the smoke stack really started drawing and the pop valve kept going off and we'd hafta stop and fill the boiler (Penberthy Injectors) to keep the crown sheet wet. Closing both dampers helped some but the stops let me get off that durn bucket.
Seems like when he did a real busy shift, the motor would pull down to 600 rpm but would rev back ok.