The conjoined banana twins are normally called double-fingers. In that case you get six locules joined along the vertical axis of two of them.
Like so.
Gabe knows exactly what causes this (I don't) but it's fairly common in some varieties. I had an entire bunch of Dwarf Cavendish fruit turn out doubled except for two bananas, and I'd expect it's a chance mutation since the pups of that plant put out normal bunches.