Twin ropes are used in pairs, just as half ropes are, but are clipped like a single rope, with both strands going through every piece of protection together. They are tested in CE drop towers as a pair, instead of individually like double ropes are. Twin ropes offer increased security in situations where damage to your rope is a possibility. Twin ropes are marked with two linked circles inside of a larger circle.
Some Edelweiss ropes are rated for not just one option (single, half, or twin) but instead are dual or triple rated for use in a variety of applications.