A split footrest means the lower portion of the footrest is likely smaller than the top part. The lower part of the footrest is generally to support your feet. A stretchy slipcover may conform to the split footrest’s shape quite well. If not, you could always modify the cover by using upholstery tacks on the underside of the footrest. Keep a little give on the footrests so any movement of your feet doesn’t rip the fabric.