It's not possible you would see stairs when there's a jetway on an FSDT airport, since they have jetways that GSX can recognize, so it won't display front stairs, just the rear one. This, of course, assuming you haven't customized the parking yourself and un-set the "Parking has a jetway" option, which was automatically and correctly set to enabled (if the parking has a jetway) by GSX.
The 737 NGX configuration that comes by default with GSX has only one front and one rear door so, on an airport with proper FSX jetways (like all FSDT airports), if you are on a parking with a jetway, you will see ONLY one stair, on the rear door. This can be optionally disabled too, with the relevant option in the GSX parking customization edito.
Instead, if you go to a 3rd party airport with static/fake jetways, that GSX cannot obviously recognize (since they don't exists as jetways, but are just generic polygons as if they were part of a building), it's normal you will see the front stairs too, because GSX cannot automatically detect the jetway. Which is why, the parking customization editor in GSX allows you to set the "Parking has a jetway" option, in order to help GSX detect a jetway on airports with fake/static jetways.