Food items don't stack across commanders?

[mivayan]

I have three commanders with one winebag each with aggregate supply <75

I dont fully understand what the problem is... you mean "supplies" is still less than 75? That is normal, the items dont affect the supplies at all, they affect the "supply usage". An army of 75 humans-sized units with 3 wineskins should have a supply usage of around 0. Bring the army to a lab and put one of the wineskins in the lab, you should immidately see the supply usage increase by 25.