Here's a chart from Tauton (Threads magazine) about twin needles and machine settings, uses, etc.
http://www.taunton.com/threads/pdf/TwinNeedleChart.pdf
and also some info on bobbins that I happened to find at the same time (has nothing to do with twin needles)--
http://www.superiorthreads.com/educa...-about-bobbins