In general, two pumps is going to be better than 1, but if you're getting enough w/ 1 pump and it's occasional, then I think you're ok.
The only thing I'd be worried about is the "regulation" of your supply. Usually this occurs somewhere in the 3-6 month range, and may take longer when you have oversupply. Usually your pumping output decreases as this occurs.
If you aren't able to pump regularly twice a day when this regulation occurs, you may see you aren't pumping as much. At this point you'll likely need to pump twice more regularly.