[KTL]

The detents don't really "lock" the selection of the gears. They just provide a spring pressure on the metal pills to hold the pill against the notch/relief in the shift rods. It keeps the rods from moving by themselves when a gear isn't selected on that rod.

You can overcome the pressure of the detent spring by simply pushing on a rod by hand and you will select whatever gear you want. So, you should be able to select two gears at a time no problem by using the individual fork rods, not the main/shifter shift rod, for 1-2 and 3-4

Here's some basic info. from the '72-'83 manual. The last image shows you a cross section of how the vertical detent should be engaging the rods