Both processes DO work at the same time.
However, on Boarding, the full loaders come from a distance so, they'll start to load as soon as they get there, which of course can change a lot depending how well (or bad) the scenery AFCAD is made, for example if it has (or not), vehicle parking close to the terminal, as they should be.
Passengers will start boarding as soon as you open the main passenger exit.
So, if you *really* want to have luggage loaded before or at the same time as passengers, don't open the passenger exit until loaders have arrived and started their job.
Not sure how better or realistic would be teleporting the loaders immediately, just because the scenery hasn't been made with the proper attention to ground services, by placing the vehicle parking spots too far away from the terminal (or not placing them at all, meaning GSX will spawn the loaders from a fuel parking or, in case *that* one is missing too, from a random parking)