Double banking

Double banking, sometimes hyphenated as double-banking, is the process by which two episodes of a programe are filmed simultaneously by two separate filming units. It therefore allows for the recording of more episodes within the same period of time. Though the concept of second unit photography has existed for a long time in the history of Doctor Who, second units were used in the 1963 version of the program mostly to hasten the pace of location filming. Double banking implies simultaneous filming of two entire stories.

History
The idea for the process germinated with the experience of filming The Long Game during the first series of the BBC Wales version of the programme. Because the production team had increasingly slipped behind schedule during the course of the series' principal photography, The Long Game was partially filmed alongside The Empty Child and The Doctor Dances. The entirely studio-bound nature of The Long Game, as well as the presence of temporary "third leads", Bruno Langley and John Barrowman, narrowly allowed the production team to complete all three episodes.

For the second series, head writer Russell T Davies wanted a less harrowing filming schedule. He therefore devised the concept of the "Doctor-lite episode" — a story which did not greatly require the presence of the programme's lead actors. One of the teams would principally work in studio, while the other would mainly work on location. This then allowed two complete production units to function simultaneously.

It became an absolute necessity, however, when the BBC added a 2005 Christmas special to their order for the 2006 series, and even more vital when they tacked on a 2006 Christmas special for their Series 3 order. This meant that, unlike Series 1, each subsequent series had 14 episodes. Thus, what had started as a way to better accommodate the filming of 13 episodes now became the only practical way to film a 14-episode series.

Double banked episodes
As of the fourth series, the following episodes have been double banked:

Unintentional

 * The Long Game/The Empty Child/The Doctor Dances. These episodes were not fully double banked.  Game was forced to enter production during the filming of the two Steven Moffat-penned episodes in order to finish shooting the series on schedule.

Intentional

 * Love & Monsters/The Impossible Planet/The Satan Pit. Though the original plan was for Monsters to film alongside Fear Her, delays in the Matt Jones-written two-parter changed the episodes with which Monsters would be paired.
 * Blink/Human Nature/The Family of Blood. These three were always scheduled against each other, and production followed the plan.
 * Midnight/Turn Left. The idea of a companion-lite and  a Doctor-lite episode being filmed against each other was inherent in the writing of the episodes, and production followed that plan.

Etymology
The term apparently derives from the game of croquet, wherein two games played simultaneously are said to be "double banked".