A drawing consisting of two lovers' names beneath a sketch of an umbrella, as seen in post #649157 or post #291423. It is basically the Japanese equivalent of writing the lovers' names inside a heart as done in the West.
For two people sharing an actual umbrella, use the tag shared umbrella.
By the way, it's not written "愛愛傘" with "ai", the kanji for "love", but is variously written "相合傘", "相合い傘", "相々傘", and "相相傘". So the translation isn't really "love love umbrella" but more like "togetherness umbrella".