If the Stubler mentions that (I didn't look it over real well), then that's just asking for breakage.
If the doubler is in LO with the rear case in HI, all the torque multiplication from the 1st gear reduction will be on the coupler shaft. At a mere 1" spline diameter, it is not up to handling that much torque.
There would NO benefit from this either, the doubler and rear case have the same ratio, so best just put the rear case in low first.
Any combination of 1350 and 1354 cases can be used, although its FAR easier to find a manual 1350 case than a 1354 to use as the doubler box (plus, cutting apart a rare 1354 manual case is likely to make you some enemies too lol)