This is no science and a rather simple conversion but yet I haven`t seen any threads on this topic. I think it could be useful for some bag makers and upholstery folks. Split needle bar machines (you can disengage and reengage the entire left and right needle bar on these machines) are rather expensive so converting a plain Singer type double needle machine into a split needle machine (disengage and reengage just the left or right needle holder) is a rather cheap alternative for low volume