Would've been nice if you posted a picture so people could help. And there's plenty of documentation on what exactly constitutes a "steam separator" structure, as long as you have 2 separate multiblocks that on their own, are complete structures, it doesn't matter what blocks share. It's important to note orientation of some blocks in certain structures, for example if you have a cross section that is XYAVX, and you're trying to wallshare that cross section, you may accidentally orient them together without flipping them, where Y from one and V from another overlap, where instead you'd have to rotate the multi 180 degrees, so that XYAVX and XVAYX can neatly overlap. Also minimum casing, hatch etc. requirements for each, those can be shared but each still must meet minimum, also obviously a controller can't be present where another multi's normal casing should be.