shared scarf
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
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 (couple muffler) is used when exactly two people are sharing a scarf together. Otherwise, the tag 連マフラー (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 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.
This tag implicates scarf and shared_clothes (learn more).