Conjoined twins Marieme and Ndeye Ndiaye seem to be as similar as any two people can be, physically and genetically, but to their dad, Ibrahima Ndiaye, says the 6-year-olds couldn't be more different.

One twin is an extrovert while the other is reserved — some scientists think conflicting traits may prompt a fertilized egg to split in two Conjoined twins Marieme and Ndeye Ndiaye seem to be as ...