This give an idea of how I operate the dual gate using GSX. This is how it works for me, so it should be a starting point for operating the airbridge. (Note that I try and refer to this as an airbridge, as is the local custom, but you'd probably know it as a jetway.)
It is tricky to make out my selections here, so here's a quick outline:
Rather than selecting 'request deboarding' first, which only uses one airbridge in my case, I choose 'operate jetways' first, and dock each in turn. The front one to door 1, of course, and aft further back.
Then once docking is finished, I can 'request deboarding' as normal, and let it do it's thing.