get a second person to help you. or get very good at holding the fabric with your toe! lol
I always have my husband hold one side I hold the other side and then I just slit down the middle.
OR another way to do it is to hold the fabrics TOGETHER and clamp it down to something on one end and hold the other end taught in your hand. Put the ripper between the two layers of fabric and move forward SLOWLY! as long as its sharp it shouldn't take much more than a touch.