Sorry, but catering and boarding at the same time it's quite unusual procedure.
Also, on some airplanes (the A318/319, for example) it's not even physically possible to have a baggage loader together with a catering service, because there's not enough space.
And, a reason why some services are not done at the same time, it's also because this way we minimize the chance for vehicles to run into each other, since they are not aware of each other, and trying to add this, in addition to create performance problems, it will likely cause all sort of problems like traffic deadlocks on the already cramped space in the parking.