A [[scarf]] worn by at least two persons. Sharing a scarf usually means at least mutuality between the wearers.

On pixiv, the tag "二人マフラー":http://dic.pixiv.net/a/%E4%BA%8C%E4%BA%BA%E3%83%9E%E3%83%95%E3%83%A9%E3%83%BC (couple muffler) is used when exactly two people are sharing a scarf together. Otherwise, the tag "連マフラー":http://dic.pixiv.net/a/%E9%80%A3%E3%83%9E%E3%83%95%E3%83%A9%E3%83%BC (group muffler) is used on pixiv to denote images where a single scarf is used by 3 or more people.

"あったかあったか":http://dic.pixiv.net/a/%E3%81%82%E3%81%A3%E3%81%9F%E3%81%8B%E3%81%82%E3%81%A3%E3%81%9F%E3%81%8B is a reference to a particular scene in K-ON! Episode 7 "Christmas!" where [[Hirasawa Yui]] notices her sister [[Hirasawa Ui]] is missing her scarf and "she decides to share half of her scarf with her":[http://k-on.wikia.com/wiki/File:Yui_and_sis.png].
二人マフラー ダブルマフラー 2人マフラー 連マフラー あったかあったか