So for some reason you attached 2 seperate files. still had to save them to my account. I made the body in one of the model's a component, and then inserted the other model with the other body as a component, and unlinked it. At this point the model is in the state I was suggestion you up load the model as.
So I'm guessing from this (just a guess) that you don't know how to use components yet. You can put components together with joints. here is a screen cast showing how to do it with your file. (I didn't know exactly where, but you should be able to figure things out with this.
FYI-the suggestion to use align to put the components together, and then use an as-built joint is crap. You use as-built joints when components have been modeled in their correct position from the start. the work flow using align and then an as-built joint will cause you nothing but grief down the road. not a good modeling practice in fusion.
also attached is your file with the joint. you;ll note in the screen cast that I temporally allowed a position capture so the components would remain where they were during the joint operation, but once I was done I delete the position capture. it's a good idea to not collect those in your timeline.
https://knowledge.autodesk.com/community/screencast/242479c6-67ce-4871-a349-f4b25ddc74dc