Since this was once a duplex, it seems reasonable to think that possibly there are two separate water supplies to the house, but if that is true, there should be two meters. Is it possible the city has forgotten one of them? In any event, the irrigation line must have a back flow prevention device to protect the city water supply. I think you have some exploring to do to see if there is another meter buried somewhere. I see no advantage of joining the two lines. I would be concerned about these pipes being galvanized and becoming corroded. 3/4" is fairly small for both domestic water supply and irrigation, depending on size of the house and size of the irrigation system as well as the condition of the pipe. If you feel you need more water supply to the house, then you might be in line for a new supply line from one of the meters with a back flow where the irrigation splits from the domestic. I would consider a 1" line. Meter size would not have to be changed.