Two handed fly lines are specifically designed for use with longer double hand salmon and switch rods. Double hand lines are usually shooting heads or WF fly lines with integrated running line. If you use shooting head, you need to have running line as well. Spey, scandi and skagit styles are the three most common casting styles and all of them have specific tapering designs. Some scandi and most skagit lines require a polyleader or tip to function properly. Double hand lines are manufactured in many different densities from floating to extra fast sinking.