2 x 757-300s will be based at LGW for the winter, with 2 x 767-300s at Manchester.
Some days only one of the two at each base operate a flight, some days neither operate any flights, but broadly speaking they have their own flying lines and 4 aircraft are needed for the programme.
As flyboyTC says, there are many days in the season where 2 x B753s from LGW and 2 x B763s from MAN operate at almost exactly the same time, so all 4 aircraft are definately required.
As usual with the winter season, it really depends which week you look at.