Are 2 INSERT statements guaranteed to complete sequentially?
It is not guaranteed if they are separate requests that are not in the same database transaction. Given the architecture of modern web apps, if they are separate requests I can almost guarantee that they are not in the same transaction and so are in no way dependant on each other.

A connection-pooled application is likely to accept the two requests with different threads, so even if the requests are issued by the client in order, the second request could insert into the database before the first if execution of the first was slower.