What do you mean by "combine these citations."
The two citations are basically for the same offense. The cop who gave the VC5204(a) citation apparently gave you some benefit of the doubt. Until you get this taken care of, you could easily get a new ticket every day or every hour.
I assume the same cop didn't give both tickets, and you probably won't get duplicates from the same cop. Frankly, I think most cops wouldn't bother ticketing for an expired registration on a car parked on private property, but maybe your city is so free of crime that the cops have little else to do (of course, if the car is parked on the street in front of your home, that's a different story). Until you can get this taken care of, I suggest you park it in such a way that the expired registration isn't readily visible to a passing cop.