Two questions:
1. Do both refineries have equal demand from different cities? If not, it will divide based on this surely.
2. Are you trying to connect two refineries to a single city? If so, this causes issues. A single refinery should be sufficient for a city for a long time, I have not myself got into a situation where a city requires over 400 goods.
I may be misreading though, been a long week, so apologies if I am way off 🙂