If there is no levi present in the synagogue, the second aliyah is given to the kohen who received the first aliyah. (Shulchan Aruch Orach Chaim 135:8)
Now, originally only the first and last olim recited the blessings preceding and following the reading, respectively. All other olim recited no blessings at all, and simply continued reading from where the previous oleh left off. (Megillah 21b)
That being the case, where no levi was present, how would one differentiate between the kohen's first and second readings? Did there use to be a different practice that was followed in such a situation?