We'll start with evidence from the Noah's Ark Circus arc. First, we have when Baron Kelvin first met Vincent and Ciel. Throughout the flashback in both the anime and manga, the watcher/reader can only see one Ciel per panel. Notice how in the anime, in the first frame, the visible Ciel is on the left, while the one in the right side is hidden behind a woman's skirt. In the next frame, the child in the right appears, but the child on the left is now hidden behind a plant pot.
Then here we see Vincent Phantomhive which a child Ciel. Take note of the visible Ciel holding his father's left arm, and how Vincent is seemingly holding and looking at someone in his right side but was obstructed from view by a woman's skirt.
In the next panel of the manga, Vincent turns around when he was called. The visible Ciel in this page is the same one from the previous panel because he is still clutching the left arm. The right side, however, is blocked by the hand, thus obscuring the brother in the right once again.
In Kelvin's Memories, we are unable to see two children at the same time because of conveniently placed vases, dresses and hands, but we can assume that there was two different figures with Vincent at the time.
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Two Ciels Theory - Black Butler Conspiracy
FanfictionTwo Ciel's Theory - A Confirmed Black Butler Kuroshitsuji Conspiracy. ・⎯♔✦♚✧♞★ 𝕬𝖑𝖑 𝕳𝖆𝖎𝖑 𝕿𝖍𝖊 𝕿𝖜𝖔 𝕮𝖎𝖊𝖑𝖘 𝕿𝖍𝖊𝖔𝖗𝖞 ★♞✧♚✦♔⎯・ It is a Kuroshitsuji Conspiracy, it is also called the Double-Ciel theory. This theory states that there ar...