I spent several decades in a rolling mill, (one of many in the Steel Company of Canada), and a double deck version would be difficult to service, getting ingots to be heated and rolled, and then transported from an upper level to some sort of system to get it to the next mill in the process.
Supporting the pits needed for heating the ingots, along with another level of the overhead cranes that handle them, let alone another rolling mill, slab scarfer, slab trimmer and slab stamper, never mind the stackers and overhead cranes for loading them on suitable cars to take them to the next phase of the operation.
The mill in which I worked periodically lengthened the building in order to add more soaking pits, then doubled the cable-type buggy tracks which delivered the ingots to the rolling mill.
With well over 140 tons in each of the 36 soaking pits, were were rolling well over 3 million tons a year.
...a couple of photos...
My suggestion to you would be to see if you can find another one of the kits that you have, then add it on to the one that you already have.
If an additional kit cannot be found, you can get all sorts of styrene material from Evergreen, which will allow you to pretty-well replicate the building that you have...so that you can connect it, making it into a longer structure.
Their offerings include sheet material in various thicknesses with corrugations on at least one side, and all types and sizes of structural steel members in styrene, which is very easy with which to work, especially since you already have a structure that you can fairly easily copy.
Properly done, it will look a lot more prototypical than a two story rolling mill.
Wayne
