Cheap Ro Zeny gives us good chances, and we can make full use of it. If there can be 2 treasure boxes up at once, then you may be right. We have not seen 2 up at the same time, though. So far from 4 days of camping: Small brown boxes, about 10 or so medium boxes, and no gold boxes.
We have seen both up at the same time. We was surprised to find this out ourselves while camping for sirocco mask, so we camped both boxes to see if they both could spawn the gold or just one. So far, the one that has only three spawn points around the level 40 maul is the only one we have seen turn gold.
Again though, it does not mean the other box will not turn gold, it just means after all our camping and seeing the south box turn gold three times we have never seen the other box do the same. We think the other box has around 4 or 5 spawn points though.