injector pair 30 of cooperating injector mechanisms 32 is positioned such that one is positioned on each side of vertical axis 18 and is movable along with first track 20 between an engaged positioned and a disengaged position in fig3 first injector pair 30 is in the engaged position second injector pair 36 of cooperating injector mechanisms 30 is positioned such that one is positioned on each side of vertical axis 18 and is movable along with second track 24 between an engaged positioned and a disengaged position in fig3 second injector pair 36 is in the disengaged position when first injector pair 30 is in the engaged position and in contact with tubing 34 second injector pair 36 is in the disengaged position while second injector pair 36 is in the disengaged position injectors 36 are removable from second track 24 for repair or replacement when first injector pair 30 becomes worn or requires service or replacement it is moved to the disengaged position by expanding hydraulic cylinders 16 attached to first track 20 and second injector pair 36 engage tubing 34 by contracting hydraulic cylinders 16 attached to second track 24 h this allows continued use of tubing.