kaitzi was the one who brought it to my attention, asking "Did Mary have twins?" My first thought was that perhaps it was Elizabeth holding the infant John the Baptist, but when I looked closer, I saw that the second baby had a halo with a cross, which clearly marked him as Jesus. (The halo with a cross was a common motif in Byzantine depictions of Jesus, so it makes sense that Giotto would use it in this work, which is very much in the Byzantine tradition.) So I told him I'd look into it, and after a little digging I turned up this passage in a long essay (unattributed, alas, but supposedly out of copyright) about Giotto's work:
Below the shelf of rock on which the Virgin's bed is laid, Giotto
has, according to the custom of the time, represented another
episode of the scene; two women, very lifelike in action and
expression, are engaged in washing and swaddling the Divine
So there you go. There are two Jesuses because that was the custom of the time. The question that now occurs to me is, who are the women? Did Mary have au pairs?