To save money, the games that had a hi/lo shifter usually only had a switch on one side.
It wasn't always consistent as to whether the switch was on the hi or lo side.
Most people who MAME cabs with this type of shifter end up adding a second switch for the other gear (so both gears have switches)
or they wire an extra input to the normally closed tab on the current switch.
Doing it the second way results in one or the other contact being closed at all times, so you'd have to disconnect it or add an on/off switch to break the connection while mapping other controls.
There is probably a way to change it in the code, but I've never looked into it.