I took my usmle step 1 today and I did not have any sequential item either.
I am so glad to hear that somebody else had the same experience because I started wondering about the craziest things: such as I did not realized they were linked and I have clicked the "next" at the top instead of the "submit" button so I have missed the second part and they will give me both answers wrong.
Bottom line I think I will not feel totally relieved until I will know my score... does it really takes 3 to 4 weeks?
Thanks