For 60 years, Doctor Who has seen its titular Time Lord regenerating into new bodies, allowing them to continue their adventures through space and time indefinitely. However, the last of Doctor Who's three 60th anniversary specials, "The Giggle," changed the laws of regeneration forever. Instead of simply changing into Ncuti Gatwa's Fifteenth Doctor, David Tennant's Fourteenth Doctor bigenerated. The bigeneration saw the outgoing Doctor divide from the new Doctor, leaving both Doctors alive as two separate Time Lords. The moment was a huge turning point in Doctor Who history and only got bigger when showrunner Russell T Davies explained the true extent of the Doctor's bigeneration.
Source：https://www.cbr.com/doctor-who-bigeneration-returning-doctors/
