Second floor options and sealing rim joists
The standard technique to second floor framing in many SIP two-story houses is the conventional approach: place floor joists and a rim along the top of SIP walls and then deck them over, per standard platform framing. This works okay. However, realize you then need to air-seal and insulate the rim system carefully or it will be just as leaky as it is in standard 2x construction. Furthermore, in cold climates you should apply an air barrier on the inside of the insulation (inside each floor joist cavity) in order to prevent migration of water vapor through the insulation and out to the cold rim where it can condense and cause cosmetic and structural problems such as peeling paint and rot.
The best way to achieve this insulation/air barrier system at the rim is to spray at least one inch of expansive urethane foam against the inside of the rim joist. In fact, you should do this around the basement rim as well (unless you're using a SIP floor over a crawl space). You don't need to hire someone to spray this foam; you can purchase a large foam pack with two-part urethane that can be applied by anyone who understands the concept behind your air barrier system. The foam might cost $200 or more per house.
For the second floor of a two-story, here's a viable alternative: order 10-foot tall wall panels and hang your second floor off the top plate with top-flange joist hangers. This provides you with an insulated rim-the panel-and makes it a snap to air-seal this traditionally leaky site. If you can install a continuous I-joist material from one side to the other-with or without mid-span support-this approach also provides an elegant way to stiffen your entire structure. However, it also requires a little extra planning when it comes to wiring your second floor. (See "Wiring" section.)