@Javis, your setup will work. A demanding unit (city, industry,…) will look for the fastest supplier which can deliver within 20 minutes. The lucky supplier will then deliver the demanded amount of cargo. So when one supplier (your coal mine) can be the fastest supplier to ship to 2 demanding units (your 2 steel mills), it will produce and deliver. 😉
But keep in mind that this setup will work as long as the two steel mills also are the fastest suppliers to their respective cities.
