The present invention concerns an apparatus and associated process for the catalytic cracking of oil cuts in a fluidized bed. The apparatus and process can catalytically crack a conventional feed in a principal riser B and crack a secondary feed in at least one secondary riser C. The lower end of the secondary riser or risers C is open to the bottom portion of the principal riser B; the upper end of the secondary riser or risers C is open to the top portion of the principal riser B.