Bob,
On my 340, with a 5E, it is physically impossible to hold the PREHEAT/STOP switch and START switch at the same time. They are both located on opposite ends of the same rocker switch.
My manual shows (Fig 6.9.1) two separate switches on the Cabin DC Distribution panel so either my manual is incorrect or your 340 and my 320 have different panel configurations. I can't get to my boat to verify either way.