Image forming system and control apparatus

An image forming system in which a first image forming apparatus and a second image forming apparatus downstream of said first image forming apparatus are connected, comprising:

• a switching unit configured to switch between a first mode in which said second image forming apparatus forms images on first and second sides of a sheet and a second mode in which said first image forming apparatus forms an image on a first side of a sheet and said second image forming apparatus forms an image on a second side of the sheet;
• and
• a control unit configured to control at least one of said first image forming apparatus and said second image forming apparatus so that when switching from the first mode in which said second image forming apparatus forms images on first and second sides of a sheet to the second mode in which said first image forming apparatus forms an image on a first side of a sheet and said second image forming apparatus forms an image on a second side of the sheet, if said second image forming apparatus is unable to perform image forming on a second side of a sheet whose first side has undergone image forming in said second image forming apparatus before said second image forming apparatus performs image forming on a second side of a sheet whose first side has undergone image forming in said first image forming apparatus, an order of the sheet whose first and second sides undergo image forming in said second image forming apparatus and the sheet whose first side undergoes image forming in said first image forming apparatus and whose second side undergoes image forming in said second image forming apparatus is not swapped.