I have a very simple technique for making a two ply off of a drop spindle. It requires very little work or tools.
When I start to spin on my spindle I use a brightly colored leader yarn to attach my new yarn to. When I build up the cop I make sure that the leader yarn is accessible so that I can slide my newly spun yarn off of the spindle and pull the first end out from the center.
I then slide the newly spun yarn onto a knitting needle slightly smaller than the diameter of the shaft of my spindle. This way I can pull from the center and the outside at the same time to do my plying.
Then all I have to do is pull out as much single ply from each end as I can handle. Hold it taught and spin in the opposite direction. This method allows me to 1. go straight from spindle to plying and 2. I always stop plying at the very center of my singles. The only disadvantage is that I can only ply the same fiber this way. I am considering working with two cops at a time and maybe use multiple fibers.